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

Happy Head’s ingredients also generally pose lower risk of side effects than other brands. — Bestreviews, Chicago Tribune, 28 Aug. 2026

The most common side effects reported in at least 2% of participants across the two trials was headache, nausea and diarrhea. — Annika Kim Constantino, CNBC, 27 Aug. 2026

Her delirium was probably a side effect of her medication, Golding says. — Michael Collins, USA Today, 27 Aug. 2026

The term describes how one’s actions have side effects that might be more valuable than the actions themselves, like a bee pollinating the world’s flowers while making honey for its hive. — Christopher Cox, Vanity Fair, 26 Aug. 2026

Word History

First Known Use

1814, in the meaning defined above

Rhymes for side effect

• aftereffect
• interconnect
• affect
• aspect
• bedecked
• bisect
• collect
• connect
• correct
• deflect
• detect
• direct

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