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

Approved since 1963, the vaccine is very safe, and side effects are usually mild and go away on their own, the agency says. — Meg Tirrell, CNN Money, 13 Sep. 2026

Some side effects reported included dry mouth, trouble peeing and nausea. — Helen Carefoot, Flow Space, 11 Sep. 2026

Antidepressants can be helpful for moderate to severe depression, but they are typically not warranted for mild depression, as the side effects might outweigh any potential benefits. — Andrew McLean, The Conversation, 10 Sep. 2026

Patients report little pain and few side effects, often returning to ordinary activity the day after the procedure. — Kristen Jordan Shamus, Freep.com, 10 Sep. 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