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

Having shown success during in vitro tests, this process could soon allow for enhanced drug delivery within the body and reduce the side effects of chemotherapy drugs. —Jack Knudson, Discover Magazine, 8 May 2025

However, experts say there are ways to avoid some of the side effects. —Emily Burns, Footwear News, 7 May 2025

Key to making those decisions is how effective the drugs are and which side effects people might experience while taking them. —Alice Park, Time, 12 May 2025

But a fortuitous mistake ushered the ingredient into mainstream skincare when a patient prescribed oral tranexamic acid experienced skin lightening as a side effect. —Tracy Achonwa, Vogue, 8 May 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