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

Eating gelatin as a food additive is considered safe, but taking high doses in a supplement may cause side effects, and rare allergic reactions are possible. —Amber J. Tresca, Verywell Health, 20 Apr. 2026

But many people who start them end up quitting, either because obesity treatment is framed as a limited-duration intervention or because of the cost, loss of insurance or side effects. —Suzanne Nuyen, NPR, 20 Apr. 2026

Blechner suffered mightily, with side effects so severe that doctors had to halt treatment. —Caleb Hellerman, CNN Money, 20 Apr. 2026

Some early research suggests that ibogaine could help treat depression and substance use disorders in some people, but it has been shown to have serious side effects. —Jackie Flynn Mogensen, Scientific American, 20 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