adverse drug reaction

An adverse drug reaction is any harmful and unintended response that happens when a medicine is used at normal, recommended doses. The key phrase is 'normal doses': this is not about a mistake or an overdose, but about a properly taken drug still causing trouble — the unwanted price that sometimes comes with a wanted effect.

Pharmacologists often sort reactions into types. Type A ('augmented') reactions are exaggerations of the drug's known pharmacology — predictable, dose-related, and usually reversible by lowering the dose, such as bleeding from an anticoagulant. Type B ('bizarre') reactions are not predictable from the drug's main action, are not clearly dose-related, and include allergic and idiosyncratic responses, such as a penicillin rash. Further categories describe chronic, delayed, and end-of-use reactions.

Adverse reactions are tracked after approval through pharmacovigilance systems, because rare reactions may only surface once millions of people have been exposed — far more than any clinical trial can enroll. Reporting suspected reactions is how regulators detect new safety signals and, occasionally, withdraw a drug.

Causation is genuinely hard to prove for any single patient. An unwanted event during treatment may be caused by the drug, by the disease itself, or by coincidence, so clinicians weigh timing, dechallenge and rechallenge, and biological plausibility rather than assuming the drug is always to blame.