Side effect

In medicine, a side effect is an effect, whether therapeutic or adverse, that is unintended; 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 are performed for their side effects; in that case, said side effect ceases to be a side effect and is now an intended effect. For instance,

Frequency of side effects

The World Health Organization and other health organisations characterise the probability of experiencing side effects as:^[1][2]

• Very common, ≥ ¹⁄₁₀
• Common (frequent), ¹⁄₁₀ to ¹⁄₁₀₀
• Uncommon (infrequent), ¹⁄₁₀₀ to ¹⁄₁₀₀₀
• Rare, ¹⁄₁₀₀₀ to ¹⁄₁₀₀₀₀
• Very rare, < ¹⁄₁₀₀₀₀

The European Commission recommends that the list should contain only effects where "at least a reasonable possibility" exists that it is caused by the drug and the frequency "should represent crude incidence rates (and not differences or relative risks calculated against placebo or other comparator)".^[3] That is, the frequency describes how often symptoms appear after taking the drug, not caused by the drug. Both health care providers^[4] and lay people^[5] misinterpret the frequency of side effects as describing the increase in frequency caused by the drug.

Examples of therapeutic side effects

• Avastin), used to slow the growth of blood vessels, 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.^[8]
• Buprenorphine has been shown experimentally (1982–1995) to be effective against severe, refractory depression.^[9][10]
• 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.^{[citation needed}
  ]
• attention-deficit hyperactivity disorder (ADHD), schizophrenia, phantom limb syndrome, paroxysmal extreme pain disorder, neuromyotonia, and post-traumatic stress disorder.^[11]
• premature labor, to enhance pulmonary maturation of the fetus.^[12]
• angioedema and severe allergic reactions due to its strong antihistamine properties.^[13]
• restless leg syndrome.^[14]
• Hydroxyzine, an antihistamine, is also used as an anxiolytic.^{[citation needed]}
• preeclampsia.^[12]
• Methotrexate (MTX), approved for the treatment of choriocarcinoma, is frequently used for the medical treatment of an unruptured ectopic pregnancy.^[15]
• The
  SSRI medication sertraline is approved as an antidepressant but delays conjugal climax in men, and thus may be supplied to those in which climax is premature.^[16]
• Sildenafil was originally intended for pulmonary hypertension; subsequently, it was discovered that it also produces erections, for which it was later marketed.^{[citation needed]}
• Terazosin, an α₁-adrenergic antagonist approved to treat benign prostatic hyperplasia (enlarged prostate) and hypertension, is (one of several drugs) used off-label to treat drug induced diaphoresis and hyperhidrosis (excessive sweating).^[17][18]

Examples of adverse effects

• allergic reactions, hives, swelling of the face, aching muscles and gastrointestinal discomfort.^[19]
• Feverfew – pregnant women should avoid using this herb, as it can trigger uterine contractions which could lead to premature labour or miscarriage.^[20]
• hay fever.^{[citation needed}
  ]

• Adverse drug reaction (ADR), a harmful unintended result caused by taking medication
• Combined drug intoxication
• Conservative management
• Drug-drug interaction (DDI), an alteration of the action of a drug caused by the administration of other drugs
• Paradoxical reaction, an effect of a substance opposite to what would usually be expected
• Pharmacogenetics
  , the use of genetic information to determine which type of drugs will work best for a patient
• Unintended consequences

References