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.
The intention of peptide users is to have the muscle-building effects of anabolic steroids without the side effects of infertility, oily skin, acne, liver inflammation, and so on. — Gavin Francis, The New York Review of Books, 24 Oct. 2026 If a treatment isn’t helping or is causing significant side effects, an MS specialist can reassess the approach and help identify a more appropriate medication, Ransohoff added. — Melissa Rudy, FOXNews.com, 24 Oct. 2026 For Bobby, the side effects were the price of getting his life back and having more time with our boys. — Stat Staff, STAT, 23 Oct. 2026 In some cases, Keytruda can result in serious organ inflammation, and previous attempts to combine the immunotherapies with other drugs have not only failed to improve cancer recurrence outcomes, but resulted in worse side effects. — David Cox, NBC news, 23 Oct. 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 27 Oct. 2026.

Copy Citation

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

More from Merriam-Webster on side effect

Thesaurus: All synonyms and antonyms for side effect

Britannica.com: Encyclopedia article about side effect

Love words? Need even more definitions?

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!

Last Updated: - Updated example sentences

More from Merriam-Webster

Word of the Day

Listen to the pronunciation of milieu

See Definitions and Examples »

Popular in Grammar & Usage

See More

How to Use 'Who' vs. 'Whom'

Merriam-Webster’s Great Big List of Words You Love to Hate

'Affect' vs. 'Effect'

'A' or 'an'? What about before 'h'? An Indefinite Article Guide

Why is '-ed' sometimes pronounced at the end of a word?
See More

Popular in Wordplay

See More

The Longest Long Words List

9 Words That Sound Like Insults But Aren't

12 Lovely and Unusual Words for the Natural World

Why do we 'bury the lede?'

'Za' and 9 Other Words to Help You Win at SCRABBLE
See More

Popular

See More

How to Use 'Who' vs. 'Whom'

The Longest Long Words List

9 Words That Sound Like Insults But Aren't
See More

Games & Quizzes

See All
Play Quordle: Guess all four words in a limited number of tries. Each of your guesses must be a real 5-letter word.
Quordle Can you solve 4 words at once?
Play Blossom: Solve today's spelling word game by finding as many words as you can using just 7 letters. Longer words score more points.
Blossom Pick the best words!
Play Missing Letter: a crossword with a twist. Each of the 25 puzzle words start with a different letter of the alphabet. Which letter is missing?
The Missing Letter A daily crossword with a twist
different halloween monsters in silhouette
A Monstrous Vocab Quiz Words That Go Bump in the Night
See All