Medicines sometimes harm the people they are meant to help. An adverse reaction is a harmful or unintended response to a medication that occurs at normal doses used for prevention, diagnosis, or treatment.

Reactions range from mild rashes and nausea to life-threatening anaphylaxis or organ damage. Some appear within minutes; others emerge only after weeks of exposure. Clinicians record them, report serious cases to regulatory systems, and adjust therapy accordingly. Distinguishing a true adverse reaction from coincidental symptoms requires careful history and sometimes rechallenge or specialist testing.

Risk factors include age, genetics, concurrent drugs, and underlying disease. Clear documentation protects the patient from re-exposure and contributes to broader drug-safety knowledge. Every prescription carries some possibility of an adverse reaction; vigilance keeps that risk manageable.

• Harmful unintended response to a medicine at normal doses
• Ranges from mild to life-threatening
• May appear immediately or after prolonged use
• Requires documentation and often therapy change