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What is a Drug Interaction?


Updated February 21, 2011

Question: What is a Drug Interaction?

Drug interactions occur when one medication, chemical or drug taken affects another. Substances that participate in drug interactions may be prescription medication, illegal or recreational substances, chemicals that naturally occur inside the body and/or chemicals ingested into the body by eating, breathing or through the skin. By definition, drug interaction effects may be welcome or not.

Drug interactions change the amount and type of exposure to your medication your body gets, which can and does change its intended effect. When a drug interaction occurs, the taking of a second drug (or other chemical) has either increased or decreased the effects of one or both of them, or has created entirely new effects.

The body deals with drugs in much the same way as food, environmental pollutants and cosmetics. They are broken down by enzymes and metabolized so that they can be absorbed and eliminated. Enzymes are "assigned," so to speak, to metabolize specific substances. Often an enzyme will be capable of breaking down more than one substance. One way an interaction could occur is if two or more drugs that use the same enzyme are taken (and metabolized) at the same time. When this happens, the presences of the drugs affect each other. Enzyme-related drug interactions can be brought about in other ways, as well. For example, some drugs increase the activity of an enzyme. Others inhibit it.

Share Your Info With Your Doctor
Your doctor uses information about drug interaction when planning medication administration. That’s why, if you take more than one medication and/or herbal supplement, or you engage in recreational drug taking, it’s important to tell her about it.


Brunton, L, Parker, K, Blumenthal, D, Buxton, I. (2008). Goodman & gilman's manual of pharmacology and therapeutics. New York: McGraw-Hill.
Drug Development and Drug InteractionsU.S. Food and Drug Administration website.

Stedman’s Medical Dictionary 28th edition. Lippincott Williams & Wilkins. 2006. Baltimore, MD.

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