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

These potential side effects include irritation, infection and prolonged redness. —Angelica Stabile, FOXNews.com, 11 Apr. 2026

Hennessy said that eating too much of any nutrient risks causing unwanted side effects. —Rich Bellis, NBC news, 11 Apr. 2026

What This Means for People Taking GLP-1 Drugs Researchers hope their findings will encourage doctors and researchers to take a closer look at the side effects patients are discussing online. —Emily Kay Votruba, EverydayHealth.com, 10 Apr. 2026

To avoid unpleasant digestive side effects, start by adding 1 tablespoon (or less) to your recipes. —Kirsten Nunez, Martha Stewart, 10 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