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

For decades, midlife sleep struggles have been dismissed as an inevitable side effect of getting older. — Hanna Wickes, Fort Worth Star-Telegram, 17 July 2026

In the larger trial, side effects with Lipfendra were similar to placebo. — Robert Glatter, Forbes.com, 17 July 2026

That benefit dropped only slightly over a year, and side effects — including dizziness and diarrhea — were similar between those taking the pill or a placebo, researchers found. — Matthew Perrone, Fortune, 16 July 2026

That means no hair loss, nausea or other common side effects. — Michelle Marchante, Miami Herald, 16 July 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