Every effective drug can produce effects beyond the intended one. A side effect is an unintended secondary effect of a medication or treatment, which may be harmful, neutral, or occasionally beneficial.

Side effects range from mild nuisance to severe toxicity. They may appear early or only after prolonged use. Dose, duration, patient age, genetics, and concurrent medications influence risk. Patient information leaflets list common and serious side effects; clinicians weigh them against expected benefit when prescribing.

Reporting unexpected or serious side effects contributes to post-marketing safety knowledge. Sometimes a side effect becomes a therapeutic use in another context. Open discussion of likely side effects improves adherence and prompt recognition when problems arise.

• Unintended effect of a medication or treatment
• May be mild, serious, or occasionally useful
• Influenced by dose, duration, and patient factors
• Should be discussed and monitored during therapy