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.
Examples of therapeutic side-effects 
- Bevacizumab (Avastin) has been used against dry age-related macular degeneration, as well as macular edema from diseases such as diabetic retinopathy and central retinal vein occlusion.
- Buprenorphine has been shown experimentally (1982–1995) to be effective against severe, refractory depression.
- Bupropion, an anti-depressant sold as Wellbutrin, 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.
- Sildenafil was originally intended for pulmonary hypertension; subsequently, it was discovered that it also produces erections, for which it was later marketed.
- Carbamazepine is an approved treatment for manic depression and convulsions, but has side effects useful in treating attention-deficit hyperactivity disorder (ADHD), schizophrenia, phantom limb syndrome, paroxysmal extreme pain disorder, neuromyotonia, and post-traumatic stress disorder.
- Dexamethasone and Betamethasone in premature labor, to enhance pulmonary maturation of the fetus.
- Doxepin has been used to treat Angiodema and severe allergic reactions due to its strong antihistamine properties.
- Gabapentin, approved for treatment of seizures and postherpetic neuralgia in adults, has side-effects which are useful in treating bipolar disorder, essential tremor, hot flashes, migraine prophylaxis, neuropathic pain syndromes, phantom limb syndrome, and restless leg syndrome.
- Magnesium sulfate in obstetrics for premature labor and preeclampsia.
- Methotrexate (MTX), approved for the treatment of choriocarcinoma, is frequently used for the medical treatment of an unruptured ectopic pregnancy.
- The SSRI medication sertraline is approved as an anti-depressant, but delays conjugal climax in men, and thus may be supplied to those in which climax is premature.
- Human chorionic gonadotropin is a hormone produced during pregnancy by the cells that form the placenta. It has been used as a diet hormone as well because of the supposed positive side effects it has on weight loss.
- Boseley, Sarah (2006-06-17). "Drugs firm blocks cheap blindness cure". The Guardian (London). Retrieved 2010-05-20.
- Richard Gracer The Buprenorphine Effect on Depression. Naabt.org. February 2007
- Bodkin JA. et al. (1995): "Buprenorphine Treatment of Refractory Depression", Journal of Clinical Psychopharmacology 15:49–57. PMID 7714228
- Mood Stabilizers for Bipolar Disorder (Manic Depressive). Leeheymd.com (2003-08-01). Retrieved on 2011-08-17.
- Wing 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. PMID 20308845.
- Shen WW, Mahadevan J, Hofstatter L, Sata LS (July 1983). "Doxepin as a potent H2 and H2 antihistamine for epigastric distress". Am J Psychiatry 140 (7): 957–8. PMID 6859336.
- OFF-LABEL USE OF GABAPENTIN. Idaho Drug Utilization Review. Educational Leaflet
- Pregnancy. drugs.nmihi.com
- Medscape: Medscape Access. Emedicine.com. Retrieved on 2011-08-17.