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

Roughly 86% of the intervention group reported mild to moderate side effects, primarily during the early weeks, including headaches, fatigue, dizziness and constipation, though researchers noted that no severe medical complications occurred. — Khloe Quill, FOXNews.com, 2 Nov. 2026

Common side effects with lenacapavir include injection site reactions, headache and nausea. — ABC News, 2 Nov. 2026

If so, nail cooling is one preventative option (and can help with other nail side effects too). — Maggie Ryan, Flow Space, 30 Oct. 2026

Unlike many hair growth tonics, which contain minoxidil (an over-the-counter drug that poses potential side effects), Briogeo’s hair growth serum is powered by plant extracts and vitamins that are essential to hair health. — Sophie Wirt, InStyle, 29 Oct. 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