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

Tell your care team if side effects, cost, scheduling or the effort of following your current plan are part of the problem. — Emily Cegielski, Flow Space, 12 Oct. 2026

The company continues to collect and share data with the FDA about the side effects reported with the drug since going to market, but the agency has not requested warning-label changes. — Yuki Noguchi, NPR, 10 Oct. 2026

The researchers have reported no obvious fading or known side effects from his own tattoos after more than two years. — Rupendra Brahambhatt, Interesting Engineering, 10 Oct. 2026

Companies are already competing to develop the next generation of drugs, including treatments that could reduce side effects and better preserve muscle. — Dr. Bowman Heiden, Fortune, 10 Oct. 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