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

The most common side effects of fish oil supplements are gastrointestinal, including belching, nausea, heartburn and loose stools. —Bestreviews, Chicago Tribune, 27 Apr. 2026

The frequency of other serious side effects has also declined — myocarditis or inflammation of the heart muscle, a condition reported most commonly in teenage boys, and multisystem inflammatory syndrome in children, or MIS-C, Havers said. —Helen Branswell, STAT, 27 Apr. 2026

Many falls result from a combination of manageable factors such as balance, home hazards, vision issues, and medication side effects. —News Release, San Diego Union-Tribune, 26 Apr. 2026

Another side effect can be dry, irritated eyes because of dehydration. —Cindy Krischer Goodman, Sun Sentinel, 25 Apr. 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