side effect

noun

: a secondary and usually adverse effect (as of a drug)
toxic side effects

called also side reaction

Examples of side effect in a Sentence

Nausea and diarrhea are the side effects of the drug. the harmful side effects of using chemicals to kill insects The merger created positive side effects for both companies.
Recent Examples on the Web
Examples are automatically compiled from online sources to show current usage. Read More Opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback.
Phase 4 tracks the potential benefits and side effects of a new treatment long-term after its approval. Simon Spichak, Health, 11 July 2025 This is a reaction to rapid weight loss rather than a direct side effect of the medication. Jessica Swirble, Verywell Health, 10 July 2025 Apart from a risk of anemia, patients reported fewer side effects than standard pancreatic cancer medications. Amy Feldman, Forbes.com, 9 July 2025 Her battle was certainly not over because the radiation side effects were so intense. Alyce Collins, MSNBC Newsweek, 6 July 2025 See All Example Sentences for side effect

Word History

First Known Use

1814, in the meaning defined above

Time Traveler
The first known use of side effect was in 1814

Browse Nearby Words

Cite this Entry

“Side effect.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/side%20effect. Accessed 14 Jul. 2025.

Kids Definition

side effect

noun
: an often harmful and unwanted effect (as of a drug) that occurs along with the basic desired effect
the environmental side effects of using chemicals to kill pests

Medical Definition

side effect

noun
: a secondary and usually adverse effect (as of a drug)
toxic side effects
a side effect of drowsiness caused by antihistamines

called also side reaction

Last Updated: - Updated example sentences
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