adverse drug reaction

An adverse drug reaction (ADR) is an unintended, harmful response to a medicine taken at a normal, therapeutic dose. It is the formal name for the harms a drug can cause when used as directed, as distinct from an overdose. Even a well-designed, properly used drug can produce reactions ranging from a mild rash to organ failure.

ADRs are often sorted into types. Type A reactions are predictable extensions of the drug's pharmacology, dose-related and usually manageable by adjusting the dose, such as bleeding from a blood thinner. Type B reactions are unpredictable, not obviously dose-related, and unrelated to the known mechanism, including allergic and idiosyncratic responses; these are the harder, scarier category.

Distinguishing an ADR from a side effect can be subtle: a side effect is any unintended effect, which may be harmless or even beneficial, whereas an adverse reaction is specifically harmful. Detecting, reporting, and analyzing ADRs is the job of pharmacovigilance, which keeps watch over a drug's safety throughout its life, especially after approval when it reaches far more patients than any trial could.