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

Patients in groups that gradually increased doses of the drug saw lower side effects. —Reuters, NBC news, 6 Nov. 2025

Service members such as Partezana were sworn to silence for 43 years following the tests — until the law mandating that silence was lifted in 1996 — and given little warning about the harmful physical side effects of nuclear fallout. —Hope Hodge Seck, USA Today, 6 Nov. 2025

The girls still struggle with side effects, and travel nine hours to New York, or three and a half hours to Cincinnati Children’s for regular checkups. —Cara Lynn Shultz, PEOPLE, 6 Nov. 2025

Possible side effects may include Sweeney Todd or Company. —Daniel Fienberg, HollywoodReporter, 6 Nov. 2025

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