An adverse drug reaction (abbreviated ADR) is a term to describe the unwanted, negative consequences sometimes associated with the use of medications. ADR is a particular type of adverse effect. The term is preferred over the colloquial and imprecise "side-effect" as "side-effect" implies the potential for beneficial consequences.

While ADR is probably the most precise term to describe the concept, it is not widely used in the community since it may be perceived as jargon and because of the negative-associations with the term "drug". Alternative terms with equivalent meaning to ADR include: side-effect, adverse event, adverse effect, etc.

There are many types of ADRs:

Type A, pharmacologically predictable

Type B, bizarre and unpredictable

Type C, arising from chronic use

Type D, delayed reaction

Type E, end of dose reaction

Type F, Failure of therapy

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