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. 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,[1] 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 malignant tumours).

Frequency of side effects

The probability or chance of experiencing side effects are characterised as : [2][3]

  • Very common, ≥ 110
  • Common (frequent), 110 to 1100
  • Uncommon (infrequent), 1100 to 11000
  • Rare, 11000 to 110000
  • Very rare, < 110000

Examples of therapeutic side effects

Examples of undesirable/unwanted side effects

  • Echinacea – more than 20 different types of reactions have been reported, including asthma attacks, loss of pregnancy, hives, swelling, aching muscles and gastrointestinal upsets.[16]
  • Feverfew – pregnant women should avoid using this herb, as it can trigger uterine contractions which could lead to premature labour or miscarriage.[17]
  • Asteraceae plants – which include feverfew, echinacea, dandelion and chamomile. Side effects include allergic dermatitis and hay fever.

See also

  • Pharmacogenetics: the use of genetic information to determine which type of drugs will work best for a patient


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  15. Cheshire, William P.; Fealey, Robert D. (2008). "Drug-induced hyperhidrosis and hypohidrosis: incidence, prevention and management" (PDF). Drug Safety. 31 (2): 109–126. doi:10.2165/00002018-200831020-00002. ISSN 0114-5916. PMID 18217788.
  16. "Filagra Vs Fildena". 23 February 2017. Archived from the original on 3 July 2017.
  17. Wells, Rebecca Erwin; Turner, Dana P.; Lee, Michelle; Bishop, Laura; Strauss, Lauren (2016-04-01). "Managing Migraine During Pregnancy and Lactation". Current Neurology and Neuroscience Reports. 16 (4): 40. doi:10.1007/s11910-016-0634-9. ISSN 1528-4042. PMID 27002079.
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