drug

Introduction
Principles of drug action
- Mechanisms
  - Receptors
  - Functional macromolecules
  - Membrane lipids
  - Other types of drug action
- Fate of drugs in the body
  - Dose-response relationship
  - Variability in response
  - Adverse effects
- Absorption, distribution, metabolism, and elimination
  - Absorption
  - Distribution
  - Metabolism
  - Elimination
  - Time course of drug action
Types of drugs
- Antimicrobial drugs
- Central nervous system drugs
- Cardiovascular drugs
- Drugs affecting blood
- Digestive system drugs
- Reproductive system drugs
- Endocrine system drugs
- Renal system drugs
- Dermatologic drugs
- Drugs affecting muscle
  - Drugs that affect smooth muscle
  - Drugs that affect skeletal muscle
- Autonomic nervous system drugs
- Anticancer drugs
- Immunosuppressants

References & Edit History Quick Facts & Related Topics

Images

A pharmacist in a white coat holds a paper list while searching shelves filled with medication boxes behind the pharmacy counter.

Diagram showing how epinephrine binding to a beta-adrenergic receptor on a cell membrane activates a G-protein, which then stimulates adenylyl cyclase to convert ATP into cyclic AMP (cAMP), leading to physiological effects such as increased heart rate, blood vessel dilation, and glycogen breakdown.

A clear intravenous fluid bag hangs from a metal IV stand, with tubing attached for administering fluids or medication.

Line graph showing plasma concentration of a drug over 20 hours after oral administration, with a rapid rise during absorption, a peak, and gradual decline during elimination.

A biologist in protective clothing and a mask examines samples under a microscope in a laboratory at a pharmaceutical company in Saint Petersburg, Russia.

A plastic bottle of Tums antacid tablets tipped over, with pastel-colored round tablets spilling onto a blue surface.

A blister pack of oral contraceptive pills arranged in a circular pattern, with days of the week labeled below each pill.

At a Glance

drug summary

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Health & Medicine Medicine

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External Websites

Chemistry LibreTexts - Organic Chemistry of Drugs
Verywell Health - Adverse Reaction to a Medication or Drug
MSD Manual - Consumer Version - Overview of Medication Safety and Effectiveness
National Center for Biotechnology Information - PubMed Central - Drug DesignÂ—Past, Present, Future
Better Health Channel - How drugs affect your body

Britannica Websites

Articles from Britannica Encyclopedias for elementary and high school students.

drug - Children's Encyclopedia (Ages 8-11)
drugs - Student Encyclopedia (Ages 11 and up)

drug

chemical agent

Also known as: medicine

Written by

Irvin S. Snyder

Professor of Microbiology and Immunity, West Virginia University, Morgantown. Contributor to Modern Pharmacology.

Irvin S. SnyderAll

Fact-checked by

Britannica Editors

Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....

Britannica Editors

Last updated

Oct. 17, 2026 •History

drug, any chemical substance that affects the functioning of living things and the organisms (such as bacteria, fungi, and viruses) that infect them. Pharmacology, the science of drugs, deals with all aspects of drugs in medicine, including their mechanism of action, physical and chemical properties, metabolism, therapeutics, 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...

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