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

In that trial, the addition of an mRNA vaccine also showed no new increase in the rate of serious side effects. — CNN Money, 21 Aug. 2026

The hope is that there will be fewer side effects compared to traditional chemo, where the drug can go everywhere. — Maggie Donahue, HEALTH, 20 Aug. 2026

The most common side effects include fatigue, sore arm and chills. — Sony Salzman, ABC News, 19 Aug. 2026

Colonoscopies are considered safe and effective, but temporary side effects can include dizziness, nausea, vomiting and abdominal pain. — Hanna Wickes, Sacbee.com, 18 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