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

According to early safety data, the side effects of the procedure and the gene therapy appear similar to those seen with standard brain surgery. —William A. Haseltine, Forbes.com, 24 Jan. 2026

Bottom line Although dry, itchy skin is a frustrating problem that can arise when taking cholesterol-lowering medication, using a daily barrier repair cream that can replenish cholesterol in your skin can mitigate these side effects. —Leslie Baumann, Miami Herald, 23 Jan. 2026

Its brightening effect has even been compared to hydroquinone, one of the most effective hyperpigmentation ingredients, but with far fewer side effects and a much gentler skin profile. —Beatrice Zocchi, Vogue, 23 Jan. 2026

That push comes amid growing focus on the quality of weight loss with next-generation obesity drugs, as GLP-1 treatments have raised concerns around muscle loss, side effects and patient drop-offs. —Annika Kim Constantino, CNBC, 23 Jan. 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