side effect

noun

Synonyms of side effectnext
: 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.
That same study found users to have experienced other side effects such as nausea, flushing and fatigue. — Bruce Y. Lee, Forbes.com, 8 Aug. 2026 So much of the conversation about perimenopause focuses on how to deal with the disruptive physical symptoms and side effects. — Helen Carefoot, Flow Space, 7 Aug. 2026 While the peptide’s risks and side effects are few, and its effects on mouse metabolism compelling, no one knows how long-term use affects humans yet, or what formula or dosage would be most effective. — Los Angeles Times, 6 Aug. 2026 The side effects of the COVID-19 vaccine are generally on par with other vaccines, including tenderness at the injection site and a small chance of allergic reaction. — Mary Walrath-Holdridge, USA Today, 6 Aug. 2026 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

Rhymes for side effect

See All Rhymes for side effect

Browse Nearby Words

side drum
side effect
side-end lines
See all Nearby Words

Cite this Entry

“Side effect.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/side%20effect. Accessed 8 Aug. 2026.

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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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