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side effect

or side-ef·fect

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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.
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Origin of side effect

First recorded in 1880–85
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
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
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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.

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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.
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The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.