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

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

See also: Serendipity

Possible side effects of nicotine[6][7]

Examples of adverse effects

Main article: Adverse effect

See also


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  13. ^ Shen WW, Mahadevan J, Hofstatter L, Sata LS (July 1983). "Doxepin as a potent H2 and H2 antihistamine for epigastric distress". The American Journal of Psychiatry. 140 (7): 957–8. doi:10.1176/ajp.140.7.957. PMID 6859336. Archived from the original on 4 September 2011.
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  19. ^ "Echinacea - side effects". 3 April 2023. Retrieved 28 June 2023.
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