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Drug | Definition, Types, Interactions, Abuse, & Facts | Britannica
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class="md-content"> <div class="md-article-container template-desktop infinite-pagination"> <div class="infinite-scroll-container article last"> <article class="article-content container-lg qa-content px-0 pt-0 pb-40 py-lg-20 content md-expanded" data-topic-id="171942"> <div class="grid gx-0"> <div class="col-auto"> <div class="topic-left-rail md-article-drawer position-relative d-flex border-right-sm border-left-sm open"> <div class="drawer d-flex flex-column open"> <div class="left-rail-section-content"> <div class="topic-left-rail-header text-truncate bg-gray-50 position-relative text-right d-flex align-items-center"> <div class="tlr-title px-20 py-15 text-left"> <em class="material-icons text-gray-400 d-lg-none" data-icon="toc"></em> <a class="font-serif font-weight-bold text-black link-blue" href="https://www.britannica.com/science/drug-chemical-agent">drug</a> </div> <button aria-label="Close" class="js-sections-close-button btn-link btn-sm btn d-lg-none position-absolute top-0 p-10 right-0" > <em class="material-icons font-26" data-icon="close"></em> </button> </div> <div class="section-content pl-10 pr-20 pl-sm-50 pr-sm-60 pl-lg-5 pr-lg-10 pt-10 pt-lg-0 bg-gray-50 clear-catfish-ad"> <div class="toc mb-20"> <div class="font-serif font-14 font-weight-bold mx-15 mb-15 mt-20"> Table of Contents </div> <ul class="list-unstyled my-0" data-level="h1"><li data-target="#ref1"><div class="pl-25"><a class="link-gray-900 w-100" href="/science/drug-chemical-agent">Introduction</a></div><div class="ml-40 toc-drawer sub-toc-drawer"></div></li><li data-target="#ref59504"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/science/drug-chemical-agent#ref59504">Principles of drug action</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref233909"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent#ref233909">Mechanisms</a><ul class="list-unstyled" data-level="h3"><li data-target="#ref60336"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent#ref60336">Receptors</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref60339"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Functional-macromolecules">Functional macromolecules</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref60340"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Functional-macromolecules#ref60340">Membrane lipids</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref60341"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Functional-macromolecules#ref60341">Other types of drug action</a></li></ul></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref233910"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Functional-macromolecules#ref233910">Fate of drugs in the body</a><ul class="list-unstyled" data-level="h3"><li data-target="#ref233911"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Functional-macromolecules#ref233911">Dose-response relationship</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref233912"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Functional-macromolecules#ref233912">Variability in response</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref233913"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Functional-macromolecules#ref233913">Adverse effects</a></li></ul></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref233914"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Functional-macromolecules#ref233914">Absorption, distribution, metabolism, and elimination</a><ul class="list-unstyled" data-level="h3"><li data-target="#ref233915"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Absorption">Absorption</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref233916"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Absorption#ref233916">Distribution</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref233917"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Absorption#ref233917">Metabolism</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref233918"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Absorption#ref233918">Elimination</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref233919"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Absorption#ref233919">Time course of drug action</a></li></ul></li></ul></div></li><li data-target="#ref233920"><div class="d-flex align-items-center"><button class="h1-link-drawer-button btn btn-xs btn-circle d-flex rounded" type="button" aria-label="Toggle Heading"><em class="material-icons font-18" data-icon="keyboard_arrow_right"></em></button><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Types-of-drugs">Types of drugs</a></div><div class="ml-40 toc-drawer sub-toc-drawer"><ul class="list-unstyled" data-level="h2"><li data-target="#ref233921"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Types-of-drugs#ref233921">Antimicrobial drugs</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref309594"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Types-of-drugs#ref309594">Central nervous system drugs</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref309595"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Types-of-drugs#ref309595">Cardiovascular drugs</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref309596"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Types-of-drugs#ref309596">Drugs affecting blood</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref309597"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Types-of-drugs#ref309597">Digestive system drugs</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref309598"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Reproductive-system-drugs">Reproductive system drugs</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref309599"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Reproductive-system-drugs#ref309599">Endocrine system drugs</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref309600"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Reproductive-system-drugs#ref309600">Renal system drugs</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref309601"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Reproductive-system-drugs#ref309601">Dermatologic drugs</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref309602"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Drugs-affecting-muscle">Drugs affecting muscle</a><ul class="list-unstyled" data-level="h3"><li data-target="#ref309603"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Drugs-affecting-muscle#ref309603">Drugs that affect smooth muscle</a></li></ul><ul class="list-unstyled" data-level="h3"><li data-target="#ref309604"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Drugs-affecting-muscle#ref309604">Drugs that affect skeletal muscle</a></li></ul></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref309605"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Drugs-affecting-muscle#ref309605">Autonomic nervous system drugs</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref309606"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Anticancer-drugs">Anticancer drugs</a></li></ul><ul class="list-unstyled" data-level="h2"><li data-target="#ref309607"><a class="w-100 link-gray-900" href="/science/drug-chemical-agent/Anticancer-drugs#ref309607">Immunosuppressants</a></li></ul></div></li></ul> <a class="toc-extra-link link-gray-900" href="https://www.britannica.com/science/drug-chemical-agent/additional-info">References & Edit History</a> <a class="toc-extra-link link-gray-900" href="/facts/drug-chemical-agent">Quick Facts & Related Topics</a> </div> <div class="tlr-media-slider pb-10 mb-30"> <a class="section-header link-gray-900 font-serif font-14 font-weight-bold mb-10 mx-10" 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Abnormalities in bcr-abl stimulate the continuous proliferation of bone marrow stem cells, causing an increase in myelogenous cells (granulocytes and macrophages) in the body and leading to chronic myelogenous leukemia (CML).</span></span> <button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"> <span class="link-blue">(more)</span> </button> </span> </div> </figcaption> </figure> </div><div class="topic-header"> <div class="d-flex align-items-top justify-content-between"> <div class="d-flex flex-column"> <div> <div> <h1>drug</h1></div> </div> <div class="topic-identifier font-16 font-md-20">chemical agent</div> </div> </div> <div class="d-none d-sm-flex flex-row"> <div class="mr-10 mb-15"> <button class="ai-ask-button btn border-2 btn-sm js-inline-ai-ask-button btn-outline-red-400 border-red-400"> Ask the Chatbot a Question </button> </div> <div class="d-none d-sm-block md-topic-tools qa-action-buttons mb-15" data-topic-id="171942"> <button class="js-tooltip btn btn-sm btn-outline-blue border pr-10 border-2" > <em class="material-icons md-icon ml-n10 my-n5 mr-5" data-icon="more_vert"></em> More Actions </button> <div class="md-more-popover popover popover-sm p-0 font-14 z-1"> <div> <button class="js-print-modal-button js-modal gtm-topic-tool btn btn-sm btn-link gtm-topic-tool font-weight-bold btn-link" data-modal="[data-topic-id=171942] .md-print-modal" > <em class="material-icons mr-5 ml-n10 my-n5 md-icon" data-icon="print"></em> Print </button> <div class="md-print-modal size-lg d-none"> <div class="md-modal-body"> <div class="h2 font-serif d-flex align-items-center pb-15 border-bottom"> <em class="material-icons text-blue mr-10">print</em> Print </div> <div class="mt-20 mb-10"> Please select which sections you would like to print: </div> <form action="/print/article/171942" method="post" target="_blank" rel="noopener"> <div class="print-box-items"> <ul class="list-unstyled"> <li><label><input class="mr-10" type="checkbox" name="sequence[]" value="0">Table Of Contents</label></li> </ul> </div> <input type="submit" class="btn btn-blue md-disabled" value="Print" /> </form> </div> </div> </div> <div> <button class="js-modal qa-cite-modal-button btn btn-sm btn-link gtm-topic-tool font-weight-bold btn-link" data-modal="[data-topic-id=171942] .md-cite-modal"> <em class="material-icons mr-5 ml-n10 my-n5 md-icon" data-icon="verified"></em> Cite </button> <div class="md-cite-modal size-lg d-none"> <div class="md-modal-body"> <div class="h2 font-serif d-flex align-items-center pb-15 border-bottom mb-15"> <em class="material-icons text-blue mr-10">verified</em>Cite </div> <div class="font-serif"> While every effort has been made to follow citation style rules, there may be some discrepancies. 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Stringer</div> <div class="editor-description font-12 font-serif mt-5 clamp-description text-black">Associate Professor of Pharmacology, Baylor College of Medicine, Houston, TX.</div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link gtm-byline font-12 byline-contributor text-decoration-underline"> Janet L. 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They write new content and verify and edit content received from contributors.</div> </a> <div data-popper-arrow></div> </div> <span class="btn btn-link editor-link p-0 qa-byline-link font-12 "> The Editors of Encyclopaedia Britannica</span></div> <div class="last-updated font-12 font-serif"> <span class="text-gray-700"> Last Updated: <time datetime="2024-11-14T00:00:00CST" >Nov 14, 2024</time> •</span> <a class="byline-edit-history" href="https://www.britannica.com/science/drug-chemical-agent/additional-info#history" rel="nofollow">Article History</a> </div></div> </div> <button class="d-flex d-lg-none btn btn-outline-blue border rounded-sm shadow-sm mobile-toc-button gtm-mobile-toc-inline-button d-none d-sm-block js-sections-inline-button module-spacing btn d-lg-none"> <em class="material-icons mr-5 ml-n10 my-n5 md-icon" data-icon="toc"></em> Table of Contents </button> <div class="d-flex d-sm-none flex-row"> <button class="d-flex d-lg-none btn btn-outline-blue border rounded-sm shadow-sm mobile-toc-button gtm-mobile-toc-inline-button js-sections-inline-button module-spacing"> <em class="material-icons mr-5 ml-n10 my-n5 md-icon" data-icon="toc"></em> Table of Contents </button> <button class="ai-ask-button btn border-2 ai-ask-button btn border-2 module-spacing btn-sm js-inline-ai-ask-button btn-outline-red-400 border-red-400 p-10 ml-5"> Ask the Chatbot a Question </button> </div> <div class="js-qf-module qf-module px-40 px-sm-20 py-15 mx-auto module-spacing font-14 bg-gray-50 rounded"> <div class="facts-list mt-10"> <div class=""> <div class="js-fact mb-10 line-clamp clamp-3"> <dl> <dt>Key People: </dt> <dd><a href="/biography/Charles-R-Walgreen" topicid="634531">Charles R. Walgreen</a></dd> <dd><a href="/biography/Thomas-Holloway-British-merchant-and-philanthropist" topicid="1350287">Thomas Holloway</a></dd> <dd><a href="/biography/Sir-Thomas-Lauder-Brunton-1st-Baronet" topicid="82311">Sir Thomas Lauder Brunton, 1st Baronet</a></dd> <dd><a href="/biography/Li-Shizhen" topicid="338673">Li Shizhen</a></dd> </dl> <button class="js-more-btn d-none btn btn-unstyled font-12 bg-gray-50" aria-label="Toggle more/less fact data"> <em class="js-content link-blue">(Show more)</em> </button> </div> </div> <div class=""> <div class="js-fact mb-10 line-clamp clamp-3"> <dl> <dt>Related Topics: </dt> <dd><a href="/science/antibiotic" topicid="27751">antibiotic</a></dd> <dd><a href="/science/anesthetic" topicid="24338">anesthetic</a></dd> <dd><a href="/science/antimicrobial-agent" topicid="28194">antimicrobial agent</a></dd> <dd><a href="/science/cardiovascular-drug" topicid="95626">cardiovascular drug</a></dd> <dd><a href="/science/givinostat" topicid="2257068">givinostat</a></dd> </dl> <button class="js-more-btn d-none btn btn-unstyled font-12 bg-gray-50" aria-label="Toggle more/less fact data"> <em class="js-content link-blue">(Show more)</em> </button> </div> <div class="text-center"> <a class="btn btn-sm btn-link p-0" href="/facts/drug-chemical-agent"> See all related content </a> </div> </div> </div> </div><div class="bg-gray-50 p-15 rounded module-spacing recent-news d-flex flex-column float-false"> <div> <h2 class="font-weight-bold font-14 m-0 d-inline"> News <span class="text-gray-600">•</span> </h2> <div class="recent-news-item first-recent-news-item d-inline"> <a class="font-14 gtm-ap-news-link" href="/news/171942/80bbe076f4ed743ebde3923dd28be004" rel="nofollow">New FDA rules for TV drug ads: Simpler language and no distractions</a> <span class="font-14 text-gray-600"> <span>•</span> Nov. 14, 2024, 6:17 PM ET (AP) </span> </div> </div> <div class="rest-of-recent-news-items"> <button class="js-toggle-recent-news d-flex btn btn-unstyled font-14 pr-10 rounded-sm mt-10" aria-label="Toggle additional news items"> Show less <em class="material-icons" data-icon="expand_less"></em> </button> </div> </div><!--[BEFORE-ARTICLE]--><span class="marker before-article"></span><section data-level="1" id="ref1"><!--[PREMOD1]--><span class="marker PREMOD1 mod-inline"></span><p class="topic-paragraph"><strong><span id="ref321586"></span>drug</strong>, any chemical substance that affects the functioning of living things and the organisms (such as <a href="https://www.britannica.com/science/bacteria" class="md-crosslink " data-show-preview="true">bacteria</a>, <a href="https://www.britannica.com/science/fungus" class="md-crosslink " data-show-preview="true">fungi</a>, and <a href="https://www.britannica.com/science/virus" class="md-crosslink " data-show-preview="true">viruses</a>) that infect them. <span id="ref126831"></span><a href="https://www.britannica.com/science/pharmacology" class="md-crosslink " data-show-preview="true">Pharmacology</a>, the <a href="https://www.britannica.com/science/science" class="md-crosslink autoxref " data-show-preview="true">science</a> of drugs, deals with all aspects of drugs in <span id="ref126832"></span><a href="https://www.britannica.com/science/history-of-medicine" class="md-crosslink " data-show-preview="true">medicine</a>, including their mechanism of action, physical and chemical properties, <a href="https://www.britannica.com/science/metabolism" class="md-crosslink " data-show-preview="true">metabolism</a>, <a href="https://www.britannica.com/science/therapeutics" class="md-crosslink " data-show-preview="true">therapeutics</a>, and toxicity. This article focuses on the principles of drug action and includes an overview of the different types of drugs that are used in the treatment and prevention of human <a href="https://www.britannica.com/science/disease" class="md-crosslink " data-show-preview="true">diseases</a>. For a discussion of the nonmedical use of drugs, <em>see</em> <a href="https://www.britannica.com/topic/drug-use" class="md-crosslink " data-show-preview="true">drug use</a>.</p><!--[MOD1]--><span class="marker MOD1 mod-inline"></span><!--[PREMOD2]--><span class="marker PREMOD2 mod-inline"></span><p class="topic-paragraph">Until the mid-19th century the approach to drug therapeutics was entirely <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="empirical" href="https://www.merriam-webster.com/dictionary/empirical" data-type="MW">empirical</a>. This thinking changed when the mechanism of drug action began to be analyzed in physiological terms and when some of the first chemical analyses of naturally occurring drugs were performed. The end of the 19th century signaled the growth of the <span id="ref126833"></span><a href="https://www.britannica.com/technology/pharmaceutical-industry" class="md-crosslink " data-show-preview="true">pharmaceutical industry</a> and the production of the first <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="synthetic" href="https://www.merriam-webster.com/dictionary/synthetic" data-type="MW">synthetic</a> drugs. <span id="ref126834"></span><a href="https://www.britannica.com/science/chemical-synthesis" class="md-crosslink " data-show-preview="true">Chemical synthesis</a> has become the most important source of therapeutic drugs. A number of therapeutic <a href="https://www.britannica.com/science/protein" class="md-crosslink " data-show-preview="true">proteins</a>, including certain <a href="https://www.britannica.com/science/antibody" class="md-crosslink " data-show-preview="true">antibodies</a>, have been developed through <a href="https://www.britannica.com/science/genetic-engineering" class="md-crosslink " data-show-preview="true">genetic engineering</a>.</p><!--[MOD2]--><span class="marker MOD2 mod-inline"></span><!--[PREMOD3]--><span class="marker PREMOD3 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="151930" data-asm-type="image"><div class="md-assembly-wrapper card-media " data-type="image"><a href="https://cdn.britannica.com/63/144563-050-25F8CC2F/pharmacist-medication-list-pharmacy-counter.jpg" class="gtm-assembly-link position-relative d-flex align-items-center justify-content-center media-overlay-link card-media" data-href="/media/1/171942/151930"><picture><source media="(min-width: 680px)" srcset="https://cdn.britannica.com/63/144563-050-25F8CC2F/pharmacist-medication-list-pharmacy-counter.jpg?w=300"><img src="https://cdn.britannica.com/63/144563-050-25F8CC2F/pharmacist-medication-list-pharmacy-counter.jpg?w=300" alt="How tsunamis form from earthquakes, volcanoes, and landslides" data-width="532" data-height="800" loading="eager"></picture><button class="magnifying-glass btn btn-circle position-absolute shadow btn-white top-10 right-10" aria-label="Zoom in"><em class="material-icons link-blue" data-icon="zoom_in"></em></button></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><a class="gtm-assembly-link md-assembly-title font-weight-bold d-inline font-sans-serif mr-5 media-overlay-link" href="https://cdn.britannica.com/63/144563-050-25F8CC2F/pharmacist-medication-list-pharmacy-counter.jpg" data-href="/media/1/171942/151930">How tsunamis form from earthquakes, volcanoes, and landslides</a><span>A pharmacist searching for the correct medication from a list behind the counter at a pharmacy.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div></figcaption></figure></div></div><p class="topic-paragraph">Drugs produce harmful as well as <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="beneficial" href="https://www.merriam-webster.com/dictionary/beneficial" data-type="MW">beneficial</a> effects, and decisions about when and how to use them therapeutically always involve the balancing of benefits and risks. Drugs approved for human use are divided into those available only with a prescription and those that can be bought freely over the counter. The availability of drugs for medical use is regulated by law.</p><!--[MOD3]--><span class="marker MOD3 mod-inline"></span><!--[PREMOD4]--><span class="marker PREMOD4 mod-inline"></span><p class="topic-paragraph">Drug treatment is the most frequently used type of therapeutic intervention in medicine. Its <a href="https://www.britannica.com/science/power-physics" class="md-crosslink autoxref " data-show-preview="true">power</a> and versatility derive from the fact that the <a href="https://www.britannica.com/science/human-body" class="md-crosslink " data-show-preview="true">human body</a> relies extensively on chemical communication systems to achieve <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="integrated" href="https://www.merriam-webster.com/dictionary/integrated" data-type="MW">integrated</a> function between billions of separate cells. The body is therefore highly susceptible to the calculated chemical subversion of parts of this <a href="https://www.britannica.com/topic/communication-network" class="md-crosslink autoxref " data-show-preview="true">communication network</a> that occurs when drugs are administered.</p><!--[MOD4]--><span class="marker MOD4 mod-inline"></span></section> <!--[H2]--><span class="marker h2"></span><section data-level="1" id="ref59504"> <h2 class="h1">Principles of <span id="ref321588"></span><a href="https://www.britannica.com/science/drug-action" class="md-crosslink ">drug action</a></h2> <section data-level="2" id="ref233909"> <h2 class="h2">Mechanisms</h2> <!--[PREMOD5]--><span class="marker PREMOD5 mod-inline"></span><p class="topic-paragraph">With very few exceptions, in order for a drug to affect the function of a <span id="ref126835"></span><a href="https://www.britannica.com/science/cell-biology" class="md-crosslink " data-show-preview="true">cell</a>, an interaction at the <span id="ref126836"></span><a href="https://www.britannica.com/science/molecule" class="md-crosslink " data-show-preview="true">molecular</a> level must occur between the drug and some target component of the cell. In most cases the interaction consists of a loose, reversible binding of the drug molecule, although some drugs can form strong chemical bonds with their target sites, resulting in long-lasting effects. Three types of target molecules can be distinguished: (1) receptors, (2) macromolecules that have specific cellular functions, such as enzymes, transport molecules, and nucleic acids, and (3) membrane lipids.</p><div class="module-spacing"> </div><!--[MOD5]--><span class="marker MOD5 mod-inline"></span> <section data-level="3" id="ref60336"> <h2 class="h3"><span id="ref126837"></span><a href="https://www.britannica.com/science/receptor-cellular-binding-site" class="md-crosslink ">Receptors</a></h2> <!--[PREMOD6]--><span class="marker PREMOD6 mod-inline"></span><p class="topic-paragraph"><a href="https://www.britannica.com/science/receptor-nerve-ending" class="md-crosslink " data-show-preview="true">Receptors</a> are <span id="ref126838"></span><a href="https://www.britannica.com/science/protein" class="md-crosslink " data-show-preview="true">protein</a> molecules that recognize and respond to the body’s own (endogenous) chemical messengers, such as <a href="https://www.britannica.com/science/hormone" class="md-crosslink " data-show-preview="true">hormones</a> or <a href="https://www.britannica.com/science/neurotransmitter" class="md-crosslink " data-show-preview="true">neurotransmitters</a>. Drug molecules may combine with receptors to initiate a series of physiological and biochemical changes. Receptor-mediated drug effects involve two distinct processes: <span id="ref126839"></span>binding, which is the formation of the drug-receptor complex, and receptor <span id="ref126840"></span><a href="https://www.britannica.com/science/active-transport" class="md-crosslink ">activation</a>, which moderates the effect. The term <em><span id="ref126841"></span>affinity</em> describes the tendency of a drug to bind to a receptor; <em><span id="ref126842"></span>efficacy</em> (sometimes called <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="intrinsic" href="https://www.merriam-webster.com/dictionary/intrinsic" data-type="MW">intrinsic</a> activity) describes the ability of the drug-receptor complex to produce a physiological response. Together, the <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="affinity" href="https://www.merriam-webster.com/dictionary/affinity" data-type="MW">affinity</a> and the <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="efficacy" href="https://www.merriam-webster.com/dictionary/efficacy" data-type="MW">efficacy</a> of a drug determine its potency.</p><!--[MOD6]--><span class="marker MOD6 mod-inline"></span> <!--[PREMOD7]--><span class="marker PREMOD7 mod-inline"></span><p class="topic-paragraph">Differences in efficacy determine whether a drug that binds to a receptor is classified as an <span id="ref126843"></span><a href="https://www.britannica.com/science/agonist" class="md-crosslink ">agonist</a> or as an <span id="ref126844"></span><a href="https://www.britannica.com/science/antagonism-drug" class="md-crosslink ">antagonist</a>. A drug whose efficacy and affinity are sufficient for it to be able to bind to a receptor and affect cell function is an agonist. A drug with the affinity to bind to a receptor but without the efficacy to elicit a response is an <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="antagonist" href="https://www.merriam-webster.com/dictionary/antagonist" data-type="MW">antagonist</a>. After binding to a receptor, an antagonist can block the effect of an agonist.</p><div class="module-spacing"> <DIV class="marketing-INLINE_SUBSCRIPTION marketing-content" data-marketing-id="INLINE_SUBSCRIPTION"><style> .student-promo-banner-wrapper { container-type: inline-size; margin-bottom: 15px; } @container (min-width: 475px) { .student-promo-banner { flex-direction: row; } .student-promo-banner-img-wrapper { margin-bottom: 0; margin-right: 10px; justify-content: flex-start; } .student-promo-banner-text-wrapper { text-align: left; margin-bottom: 0px; margin-left: 10px; } .student-promo-banner-button-wrapper { margin-right: 0; } }</style> <div class="student-promo-banner-wrapper"> <div class="student-promo-banner d-flex flex-column align-items-center bg-blue rounded p-20"> <div class="student-promo-banner-img-wrapper mb-20 mr-0 d-flex justify-content-center"> <img class="rounded" style="max-width: 100px; min-width: 80px" src="https://cdn.britannica.com/marketing/BlueThistle.webp" /> </div> <div class="student-promo-banner-text-wrapper ml-0 mb-10 text-center text-white"> <div class="h2 mb-10">Get Unlimited Access</div> <div class="h4 font-weight-semi-bold">Try Britannica Premium for free and discover more.</div> </div> <div class="student-promo-banner-button-wrapper d-flex justify-content-center align-items-center ml-auto mr-auto"> <a class="btn btn-m btn-orange" href="https://premium.britannica.com/premium-membership/?utm_source=premium&utm_medium=inline-cta&utm_campaign=black-friday-2024">Subscribe</a> </div> </div> </div> </DIV></div><!--[MOD7]--><span class="marker MOD7 mod-inline"></span> <!--[PREMOD8]--><span class="marker PREMOD8 mod-inline"></span><p class="topic-paragraph">The degree of binding of a drug to a receptor can be measured directly by the use of <a href="https://www.britannica.com/science/radioactivity" class="md-crosslink " data-show-preview="true">radioactively</a> labeled drugs or inferred indirectly from measurements of the biological effects of agonists and <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="antagonists" href="https://www.merriam-webster.com/dictionary/antagonists" data-type="MW">antagonists</a>. Such measurements have shown that the following <span id="ref126845"></span><a href="https://www.britannica.com/science/chemical-reaction" class="md-crosslink " data-show-preview="true">reaction</a> generally obeys the <a href="https://www.britannica.com/science/law-of-mass-action" class="md-crosslink autoxref " data-show-preview="true">law of mass action</a> in its simplest form: drug + receptor ⇌ drug-receptor complex. Thus, there is a relationship between the concentration of a drug and the amount of drug-receptor complex formed.</p><!--[MOD8]--><span class="marker MOD8 mod-inline"></span> <!--[PREMOD9]--><span class="marker PREMOD9 mod-inline"></span><p class="topic-paragraph">The structure-activity relationship describes the connection between chemical structure and biological effect. Such a relationship explains the <a class="md-dictionary-link md-dictionary-tt-off mw" data-term="efficacies" href="https://www.merriam-webster.com/dictionary/efficacies" data-type="MW">efficacies</a> of various drugs and has led to the development of newer drugs with specific mechanisms of action. The contribution of the British pharmacologist Sir <span id="ref126849"></span><a href="https://www.britannica.com/biography/James-Black" class="md-crosslink " data-show-preview="true">James Black</a> to this field led to the development, first, of drugs that selectively block the effects of <a href="https://www.britannica.com/science/epinephrine" class="md-crosslink " data-show-preview="true">epinephrine and norepinephrine</a> on the heart (<a href="https://www.britannica.com/science/beta-blocker" class="md-crosslink " data-show-preview="true">beta blockers</a>, or beta-adrenergic blocking agents) and, second, of drugs that block the effect of <a href="https://www.britannica.com/science/histamine" class="md-crosslink " data-show-preview="true">histamine</a> on the stomach (<a href="https://www.britannica.com/science/H2-receptor-antagonist" class="md-crosslink " data-show-preview="true">H<sub>2</sub>-blocking agents</a>), both of which are of major therapeutic importance.</p><!--[MOD9]--><span class="marker MOD9 mod-inline"></span> <!--[PREMOD10]--><span class="marker PREMOD10 mod-inline"></span><p class="topic-paragraph">Receptors for many hormones and neurotransmitters have been isolated and biochemically characterized. All these receptors are proteins, and most are incorporated into the cell <a href="https://www.britannica.com/science/membrane-biology" class="md-crosslink " data-show-preview="true">membrane</a> in such a way that the binding region faces the exterior of the cell. This allows the endogenous chemicals freer access to the cell. Receptors for steroid hormones (e.g., hydrocortisones and <a href="https://www.britannica.com/science/estrogen" class="md-crosslink " data-show-preview="true">estrogens</a>) differ in being located in the cell <a href="https://www.britannica.com/science/nucleus-biology" class="md-crosslink " data-show-preview="true">nucleus</a> and therefore being accessible only to molecules that can enter the cell across the membrane.</p><!--[MOD10]--><span class="marker MOD10 mod-inline"></span> <!--[PREMOD11]--><span class="marker PREMOD11 mod-inline"></span><p class="topic-paragraph">Once the drug has bound to the receptor, certain intermediate processes must take place before the drug effect is measurable. Various mechanisms are known to be involved in the processes between receptor activation and the cellular response (also called <span id="ref126854"></span>receptor-effector coupling). Among the most important ones are the following: (1) direct control of <span id="ref126855"></span><a href="https://www.britannica.com/science/ion-channel" class="md-crosslink " data-show-preview="true">ion channels</a> in the <a href="https://www.britannica.com/science/cell-membrane" class="md-crosslink autoxref " data-show-preview="true">cell membrane</a>, (2) <a href="https://www.britannica.com/topic/regulation" class="md-crosslink autoxref " data-show-preview="true">regulation</a> of cellular activity by way of intracellular chemical signals, such as cyclic adenosine 3′,5′-monophosphate (cAMP), inositol phosphates, or <a href="https://www.britannica.com/science/calcium" class="md-crosslink " data-show-preview="true">calcium</a> <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="ions" href="https://www.britannica.com/dictionary/ions" data-type="EB">ions</a>, and (3) regulation of <a href="https://www.britannica.com/science/gene" class="md-crosslink " data-show-preview="true">gene</a> expression.</p><!--[MOD11]--><span class="marker MOD11 mod-inline"></span> <!--[PREMOD12]--><span class="marker PREMOD12 mod-inline"></span><p class="topic-paragraph">In the first type of mechanism, the ion channel is part of the same protein complex as the receptor, and no biochemical intermediates are involved. Receptor activation briefly opens the transmembrane ion channel, and the resulting flow of ions across the membrane causes a change in the transmembrane potential of the cell that leads to the initiation or inhibition of electrical impulses. Such mechanisms are common for <a href="https://www.britannica.com/science/neurotransmitter" class="md-crosslink " data-show-preview="true">neurotransmitters</a> that act very rapidly. Examples include the receptors for <a href="https://www.britannica.com/science/acetylcholine" class="md-crosslink " data-show-preview="true">acetylcholine</a> and for other fast excitatory or inhibitory transmitter substances in the <a href="https://www.britannica.com/science/nervous-system" class="md-crosslink autoxref " data-show-preview="true">nervous system</a>, such as glutamate and gamma-aminobutyric acid (GABA).</p><!--[MOD12]--><span class="marker MOD12 mod-inline"></span> <!--[PREMOD13]--><span class="marker PREMOD13 mod-inline"></span><div class="assemblies"><div class="w-100"><figure class="md-assembly m-0 mb-md-0 card card-borderless print-false" data-assembly-id="121522" data-asm-type="image"><div class="md-assembly-wrapper card-media " data-type="image"><a href="https://cdn.britannica.com/90/123390-050-3D2D668C/Epinephrine-type-G-protein-coupled-receptor-beta-adrenergic-protein.jpg" class="gtm-assembly-link position-relative d-flex align-items-center justify-content-center media-overlay-link card-media" data-href="/media/1/171942/121522"><picture><source media="(min-width: 680px)" srcset="https://cdn.britannica.com/90/123390-050-3D2D668C/Epinephrine-type-G-protein-coupled-receptor-beta-adrenergic-protein.jpg"><img src="https://cdn.britannica.com/90/123390-050-3D2D668C/Epinephrine-type-G-protein-coupled-receptor-beta-adrenergic-protein.jpg?w=300" alt="epinephrine-stimulated cAMP synthesis" data-width="1600" data-height="1314" loading="eager"></picture><button class="magnifying-glass btn btn-circle position-absolute shadow btn-white top-10 right-10" aria-label="Zoom in"><em class="material-icons link-blue" data-icon="zoom_in"></em></button></a></div><figcaption class="card-body"><div class="md-assembly-caption text-muted font-14 font-serif line-clamp"><span><a class="gtm-assembly-link md-assembly-title font-weight-bold d-inline font-sans-serif mr-5 media-overlay-link" href="https://cdn.britannica.com/90/123390-050-3D2D668C/Epinephrine-type-G-protein-coupled-receptor-beta-adrenergic-protein.jpg" data-href="/media/1/171942/121522">epinephrine-stimulated cAMP synthesis</a><span>In cells the stimulatory effects of epinephrine are mediated through the activation of a second messenger known as cAMP (cyclic adenosine monophosphate). The activation of this molecule results in the stimulation of cell-signaling pathways that act to increase heart rate, to dilate blood vessels in skeletal muscle, and to break down glycogen to glucose in the liver.</span><button class="js-more-btn d-none btn btn-unstyled font-12 bg-white js-content" aria-label="Toggle more/less fact data"><span class="link-blue">(more)</span></button></span></div></figcaption></figure></div></div><p class="topic-paragraph">In the second mechanism, chemical reactions that take place within the cell trigger a series of responses. The receptor may control calcium <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="influx" href="https://www.britannica.com/dictionary/influx" data-type="EB">influx</a> through the outer cell membrane, thereby altering the concentration of free calcium ions within the cell, or it may control the <span id="ref126860"></span><a href="https://www.britannica.com/science/catalysis" class="md-crosslink " data-show-preview="true">catalytic</a> activity of one or more membrane-bound enzymes. One of these enzymes is <span id="ref126861"></span><a href="https://www.britannica.com/science/adenyl-cyclase" class="md-crosslink ">adenylate cyclase</a>, which catalyzes the conversion of <span id="ref126862"></span><a href="https://www.britannica.com/science/adenosine-triphosphate" class="md-crosslink " data-show-preview="true">adenosine triphosphate</a> (ATP) within the cell to <span id="ref1177916"></span><a href="https://www.britannica.com/science/cyclic-35-adenosine-monophosphate" class="md-crosslink ">cAMP</a>, which in turn binds to and activates intracellular enzymes that catalyze the attachment of <a href="https://www.britannica.com/science/phosphate" class="md-crosslink " data-show-preview="true">phosphate</a> groups to other functional proteins; these may be involved in a wide variety of intracellular processes, such as <a href="https://www.britannica.com/science/muscle" class="md-crosslink " data-show-preview="true">muscle</a> contraction, <a href="https://www.britannica.com/science/cell-division" class="md-crosslink autoxref " data-show-preview="true">cell division</a>, and membrane permeability to ions. A second receptor-controlled enzyme is phosphodiesterase, which catalyzes the cleavage of a membrane phospholipid, phosphatidylinositol, releasing the intracellular messenger inositol triphosphate. This substance in turn releases calcium from intracellular stores, thus raising the free calcium <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="ion" href="https://www.britannica.com/dictionary/ion" data-type="EB">ion</a> concentration. Regulation of the concentration of free calcium ions is important because, like cAMP, calcium ions control many cellular functions. (For more information on intracellular signaling molecules, <em>see</em> <a href="https://www.britannica.com/science/second-messenger" class="md-crosslink " data-show-preview="true">second messenger</a> and <a href="https://www.britannica.com/science/kinase" class="md-crosslink " data-show-preview="true">kinase</a>.)</p><!--[MOD13]--><span class="marker MOD13 mod-inline"></span> <!--[PREMOD14]--><span class="marker PREMOD14 mod-inline"></span><p class="topic-paragraph">In the third type of mechanism, which is peculiar to <a href="https://www.britannica.com/science/steroid-hormone" class="md-crosslink " data-show-preview="true">steroid hormones</a> and related drugs, the steroid binds to a receptor that consists primarily of nuclear proteins. Because this interaction occurs inside the cell, agonists for this receptor must be able to cross the cell membrane. The drug-receptor complex acts on specific regions of the genetic material <span id="ref126863"></span><a href="https://www.britannica.com/science/DNA" class="md-crosslink " data-show-preview="true">deoxyribonucleic acid</a> (DNA) in the cell nucleus, resulting in an increased rate of <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="synthesis" href="https://www.britannica.com/dictionary/synthesis" data-type="EB">synthesis</a> for some proteins and a decreased rate for others. Steroids generally act much more slowly (hours to days) than agents that act by either of the two other mechanisms.</p><div class="one-good-fact-module"> </div><!--[MOD14]--><span class="marker MOD14 mod-inline"></span> <!--[PREMOD15]--><span class="marker PREMOD15 mod-inline"></span><p class="topic-paragraph">Many receptor-mediated events show the phenomenon of <span id="ref126865"></span><a href="https://www.britannica.com/science/desensitization-medicine" class="md-crosslink " data-show-preview="true">desensitization</a>, which means that continued or repeated administration of a drug produces a progressively smaller effect. Among the complex mechanisms involved are conversion of the receptors to a refractory (unresponsive) state in the presence of an agonist, so that activation cannot occur, or the removal of receptors from the cell membrane (down-regulation) after prolonged exposure to an agonist. Desensitization is a <a href="https://www.britannica.com/science/reversibility" class="md-crosslink autoxref " data-show-preview="true">reversible process</a>, although it can take hours or days for receptors to recover after down-regulation. The <a class="md-dictionary-link md-dictionary-tt-off eb" data-term="converse" href="https://www.britannica.com/dictionary/converse" data-type="EB">converse</a> process (up-regulation) occurs in some instances when receptor antagonists are administered. These adaptive responses are undoubtedly important when drugs are given over a period of time, and they may account partly for the phenomenon of tolerance (an increase in the dose needed to produce a given effect) that occurs in the therapeutic use of some drugs.</p><!--[MOD15]--><span class="marker MOD15 mod-inline"></span> </section> </section></section><!--[END-OF-CONTENT]--><span class="marker end-of-content"></span><!--[AFTER-ARTICLE]--><span class="marker after-article"></span></div> <div id="chatbot-root"></div> </div> </div> </div> <div class="ai-dialog-placeholder"></div> </div> </div> <aside class="col-md-da-320"></aside> </div> </div> </div> </div> </article></div> </div></div> </div> </main> <div id="md-footer"></div> <noscript><iframe src="//www.googletagmanager.com/ns.html?id=GTM-5W6NC8" height="0" width="0" style="display:none;visibility:hidden"></iframe></noscript> <script type="text/javascript" id="_informizely_script_tag"> var IzWidget = IzWidget || {}; (function (d) { var scriptElement = d.createElement('script'); scriptElement.type = 'text/javascript'; scriptElement.async = true; scriptElement.src = "https://insitez.blob.core.windows.net/site/f780f33e-a610-4ac2-af81-3eb184037547.js"; var node = d.getElementById('_informizely_script_tag'); node.parentNode.insertBefore(scriptElement, node); } )(document); </script> <!-- Ortto ebmwprod capture code --> <script> window.ap3c = window.ap3c || {}; var ap3c = window.ap3c; ap3c.cmd = ap3c.cmd || []; ap3c.cmd.push(function() { ap3c.init('ZO4siT4cLwnykPnzZWJtd3Byb2Q', 'https://engage.email.britannica.com/'); ap3c.track({v: 0}); }); ap3c.activity = function(act) { ap3c.act = (ap3c.act || []); ap3c.act.push(act); }; var s, t; s = document.createElement('script'); s.type = 'text/javascript'; s.src = "https://engage.email.britannica.com/app.js"; t = document.getElementsByTagName('script')[0]; t.parentNode.insertBefore(s, t); </script> <script class="marketing-page-info" type="application/json"> {"pageType":"Topic","templateName":"DESKTOP","pageNumber":1,"pagesTotal":7,"pageId":171942,"pageLength":1492,"initialLoad":true,"lastPageOfScroll":false} </script> <script class="marketing-content-info" type="application/json"> [] </script> <script src="https://cdn.britannica.com/mendel-resources/3-130/js/libs/jquery-3.5.0.min.js?v=3.130.14"></script> <script type="text/javascript" data-type="Init Mendel Code Splitting"> (function() { $.ajax({ dataType: 'script', cache: true, url: 'https://cdn.britannica.com/mendel-resources/3-130/dist/topic-page.js?v=3.130.14' }); })(); </script> <script class="analytics-metadata" type="application/json"> {"leg":"C","adLeg":"C","userType":"ANONYMOUS","pageType":"Topic","pageSubtype":null,"articleTemplateType":"PAGINATED","gisted":false,"pageNumber":1,"hasSummarizeButton":false,"hasAskButton":false} </script> <script type="text/javascript"> EBStat={accountId:-1,hostnameOverride:'webstats.eb.com',domain:'www.britannica.com', json:''}; </script> <script type="text/javascript"> ( function() { $.ajax( { dataType: 'script', cache: true, url: '//www.britannica.com/webstats/mendelstats.js?v=1' } ) .done( function() { try {writeStat(null,EBStat);} catch(err){} } ); })(); </script> <div id="bc-fixed-dialogue"></div> </body> </html>