Educational Content: This article is for informational purposes only and does not constitute medical advice. Always consult a healthcare provider for diagnosis and treatment.
Definition of Adverse reaction
Medically reviewed by Dr. Otari Nergadze, Neurosurgeon | Updated: January 2026
Adverse reaction: In pharmacology, any unexpected or dangerous reaction to a drug. An unwanted effect caused by the administration of a drug. The onset of the adverse reaction may be sudden or develop over time.
Also called an adverse drug event (ADE), adverse drug reaction (ADR), adverse effect or adverse event.
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