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

Treatment for Tardive Dyskinesia Tardive dyskinesia is a movement disorder that can develop as a side effect of antipsychotics. — Heidi Moawad, Verywell Health, 26 Sep. 2026

Most new drugs try to make small improvements on existing treatments, like reducing the side effects. — Gerry Smith, CNBC, 25 Sep. 2026

Care will continue, including through clinicians who prescribe misoprostol alone, an effective regimen with more side effects or new medications being developed. — Julie F. Kay, STAT, 25 Sep. 2026

The formula’s lead ingredient, soybean ferment extract, is an exceptional resurfacing agent—improving skin’s texture, tone, and radiance without any harsh side effects. — Kiana Murden, Vogue, 25 Sep. 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