A drug interaction is a situation in which a substance affects the activity of a drug, i.e. the effects are increased or decreased, or they produce a new effect that neither produces on its own. Typically, interaction between drugs come to mind (drug-drug interaction). However, interactions may also exist between drugs & foods (drug-food interactions), as well as drugs & herbs (drug-herb interactions).

Generally speaking, drug interactions are to be avoided, due to the possibility of poor or unexpected outcomes. However, drug interactions have been deliberately used, such as co-administering probenecid with penicillin prior to mass production of penicillin. Because penicillin was difficult to manufacture, it was worthwhile to find a way to reduce the amount required.

Probenecid retards the excretion of penicillin, so a dose of penicillin persists longer when taken with it, and it allowed patients to take less penicillin over a course of therapy. A contemporary example of a drug interaction used as an advantage is the co-administration of carbidopa with levodopa (available as Carbidopa/levodopa). Levodopa is used in the management of Parkinson's disease and must reach the brain in an un-metabolized state to be beneficial.

When given by itself, levodopa is metabolized in the peripheral tissues outside the brain, which decreases the effectiveness of the drug and increases the risk of adverse effects. However, since carbidopa inhibits the peripheral metabolism of levodopa, the co-administration of carbidopa with levodopa allows more levodopa to reach the brain un-metabolized and also reduces the risk of side effects. Drug interactions may be the result of various processes.

These processes may include alterations in the pharmacokinetics of the drug, such as alterations in the Absorption, Distribution, Metabolism, and Excretion (ADME) of a drug. Alternatively, drug interactions may be the result of the pharmacodynamic properties of the drug, e.g. the co-administration of a receptor antagonist and an agonist for the same receptor.

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