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definitions

side effect

or side-ef·fect

See more synonyms for side effect on Thesaurus.com
noun
  1. 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.
  2. 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

Related Words for side effect

reaction, aftereffect, aftermath, concomitant, corollary, leftover, offshoot, spinoff, aftertaste, byproduct, sidebar
British Dictionary definitions for side effect

side effect

noun
  1. any unwanted nontherapeutic effect caused by a drugCompare aftereffect (def. 2)
  2. any secondary effect, esp an undesirable one
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
Word Origin and History for side effect
n.

also side-effect, 1884, from side (adj.) + effect (n.). Medical use, with reference to drugs, is recorded from 1939.

Online Etymology Dictionary, © 2010 Douglas Harper
side effect in Medicine

side effect

n.
  1. 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.