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Synonyms

side effect

American  
Or side-effect

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.
side effect British  

noun

  1. any unwanted nontherapeutic effect caused by a drug Compare aftereffect

  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

Etymology

Origin of side effect

First recorded in 1880–85

Example Sentences

Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.

GLP-1 users, she added, have also gravitated toward fiber to help deal with constipation, a common side effect of those medications.

From MarketWatch • May 10, 2026

What will the next scan bring when it comes to a treatment, surgery, or side effect that means this healthy, happy moment was fleeting?

From Slate • May 6, 2026

Mild-to-moderate injection site reactions were the most common side effect, but nearly all of these reactions resolved themselves within five days.

From Barron's • Apr. 20, 2026

But scientists are now raising concerns about an unexpected side effect.

From Science Daily • Apr. 18, 2026

It was not an accident or side effect.

From "Educated" by Tara Westover

Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.