In medicine, a side effect is an effect, whether therapeutic or adverse, that is secondary to the one intended; although the term is predominantly employed to describe adverse effects, it can also apply to beneficial, but unintended, consequences of the use of a drug.
Occasionally, drugs are prescribed or procedures performed specifically for their side effects; in that case, said side effect ceases to be a side effect, and is now an intended effect. For instance, X-rays were historically (and are currently) used as an imaging technique; the discovery of their oncolytic capability led to their employ in radiotherapy (ablation of malignant tumours).
Use of drugs for unapproved indications—that is, for their side effects—is termed off-label use. For instance, opioids, which are approved as palliative treatment for pain, may also be used for their euphoriant properties as anxiolytics or antidepressants, whether medically or recreationally.
Off-label use of drugs, although similar in meaning to side effects, is not to be confused with them; side effects describe the mechanism of action that a drug used off-label takes, whereas off-label use implies the desired consequences of using a drug for its side-effects.
Off-label use of drugs is legal; the manner in which prescription medication is to be used is solely at the discretion of the prescriber. However, the marketing of drugs towards unapproved indications is illegal; in fact, several pharmaceutical development firms have been fined for unapproved promotion of their products.
Read more about Side Effect: Examples of Therapeutic Side-effects
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