A side-effect is any effect other than an intended primary effect. It may or may not be expected.

In particular, the term is often used in the medical field with regard to drugs, where the primary effect is that what the drug is usually prescribed for. Although the most common sense of the term side-effect refers to undesired side-effects, in certain cases side-effects can be beneficial.

For example, many antihistamines, in addition to suppressing allergic symptoms, also induce sleepiness. This can be very undesirable if the person taking them wishes to stay awake. However, many psychiatric patients cannot be prescribed the usual sleeping pills and in this case the "side effect" can become the intended primary effect of the prescription.

An example of completely undesirable side-effects are the extrapyramidal side-effects of antipsychotic drugs. Additional drugs (which have their own side-effects) often have to be prescribed to attempt to limit these side-effects to tolerable levels.

The term is also frequently used to describe unintended effects of laws and regulations.

The term has a more specific technical use in computing; see side-effect (computer science).

See also: polypharmacy, collateral damage