Side effect

In medicine, a side effect is an unintended effect caused by a medicinal drug or other treatment's capacities or properties, and these effects are often adverse but sometimes beneficial. Herbal and traditional medicines also have side effects.

A drug or procedure usually used for a specific effect may be used specifically because of a beneficial side-effect; this is termed "off-label use" until such use is approved. For instance, X-rays have long been used as an imaging technique; the discovery of their oncolytic capability led to their use in radiotherapy for ablation of malignant tumours.