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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.

More recently, it’s become an off-label success story after doctors realized a side effect of taking the drug was hair growth.

From MarketWatch • Apr. 27, 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

The most commonly reported side effect was a decrease in neutrophils, which are a type of white blood cell.

From Science Daily • Apr. 3, 2026

The moves are a side effect of the war with Iran, which has spread across more of the Middle East, creating tighter supplies of oil and petroleum products such as jet fuel.

From The Wall Street Journal • Apr. 1, 2026

“Dizziness is a side effect of the serum. You may want to rest this afternoon. And I’m feeling well. Thank you for asking,” he says.

From "Insurgent" by Veronica Roth

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.