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. Developing drugs is a complicated process, because no two people are exactly the same, so even drugs that have virtually no side effects, might be difficult for some people. Also, it is difficult to make a drug that targets one part of the body but that doesn’t affect other parts, the fact that increases the risk of side effects in the untargeted parts.
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 malignanttumours).
Buprenorphine has been shown experimentally (1982–1995) to be effective against severe, refractory depression.
Bupropion (Wellbutrin), an anti-depressant, is also used as a smoking cessation aid; this indication was later approved, and the name of the smoking cessation product is Zyban. In Ontario, Canada, smoking cessation drugs are not covered by provincial drug plans; elsewhere, Zyban is priced higher than Wellbutrin, despite being the same drug. Therefore, some physicians prescribe Wellbutrin for both indications.
^ abWing, DA; Powers, B; Hickok, D (2010). "U.S. Food and Drug Administration Drug Approval: Slow Advances in Obstetric Care in the United States". Obstetrics & Gynecology. 115 (4): 825–33. doi:10.1097/AOG.0b013e3181d53843. PMID20308845.