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 new rules had some bad side effects and now hopefully, those holes have been patched. —New York Daily News Editorial Board, New York Daily News, 30 Apr. 2025

Drinking lemon water is generally safe, though there are some side effects to be aware of. —Sherri Gordon, Health, 30 Apr. 2025

Savchuk notes that mild redness, peeling, swelling, skin tightness, itching, and increased sun sensitivity are common side effects. —Larry Stansbury, Essence, 29 Apr. 2025

While immunotherapies also come with side effects—including fatigue, skin rashes, and, in a few patients, hypothyroidism—these are often manageable. —Alice Park, Time, 28 Apr. 2025

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