'副作用' literally means 'secondary effect' — '副' (fù) means 'secondary' or 'subsidiary', '作' (zuò) and '用' (yòng) together form 'effect' or 'function'. It refers to unintended, often undesirable effects that occur alongside the main therapeutic effect of a drug, treatment, or medical procedure. Unlike the intended benefit, side effects are usually negative (e.g., drowsiness from antihistamines or nausea from chemotherapy), though some can be neutral or even beneficial in rare cases.

This term is used almost exclusively in medical and health-related contexts — by doctors, pharmacists, patients, and in drug packaging or clinical literature. It’s not used for general unintended consequences outside healthcare (e.g., you wouldn’t call traffic jams a 'side effect' of building a new road in Chinese — use '后果' or '影响' instead). The word carries a technical, formal register and appears frequently in warnings, patient information leaflets, and consultations.