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

Loss of muscle strength, medication side effects, neurological conditions, and changes in vision or balance can all affect gait. — Katia Hetter, CNN Money, 14 Aug. 2026

Just know that other negative side effects, like a lower credit score, won’t vanish. — Ryley Amond, CNBC, 14 Aug. 2026

The Rockies are weighing the side effects of a work stoppage against the value of exposing Condon to big league pitching as the franchise focuses on building up its core moving forward. — Troy Renck, Denver Post, 13 Aug. 2026

The treatment is modestly effective with manageable side effects. — Adam Feuerstein, STAT, 13 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