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

Normally, converting coal to chemicals produces a lot of waste carbon dioxide and has other side effects that lower efficiency. —Christopher McFadden, Interesting Engineering, 4 Jan. 2026

Gopie said the radiation had new side effects, including intense headaches and a burning feeling in her skin. —Kerry Breen, CBS News, 3 Jan. 2026

Haile told Fox News Digital that there were no adverse side effects reported in the animal study, likely because the ingredients of the vaccine are proteins that have already been used in other combinations in humans, so their safety profile is already known. —Melissa Rudy, FOXNews.com, 2 Jan. 2026

Traditional options, including some antihistamines, have been the main interventions for nearly half a century, despite limited effectiveness and side effects such as drowsiness. —New Atlas, 2 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