Side effect

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
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 * 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.<ref
 * 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.