- any effect of a drug, chemical, or other medicine that is in addition to its intended effect, especially an effect that is harmful or unpleasant.
- any accompanying or consequential and usually detrimental effect: the side effects of air pollution.
Origin of side effect
First recorded in 1880–85
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
- any unwanted nontherapeutic effect caused by a drugCompare aftereffect (def. 2)
- any secondary effect, esp an undesirable one
Word Origin and History for side effect
Online Etymology Dictionary, © 2010 Douglas Harper
- A peripheral or secondary effect, especially an undesirable secondary effect of a drug or therapeutic regimen.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.