Side-effect

A side-effect is any effect other than the primary effect.

In particular the term is used 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, some side-effects can be beneficial (in certain cases).

An example of a potentially beneficial side-effect is the sleepiness that many anti-histamines[?] induce. In most people this is very much an undesirable side-effect. But many psychiatric patients cannot be prescribed the usual sleeping pills[?] and in this case the "side effect" can be a very desirable effect.

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.