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

The brand holds three patents, including its signature DermShield technology that allows skincare brands to increase the concentration of active ingredients in a product without typical side effects like irritation and redness. — Ellen Thomas, Vogue, 5 Oct. 2026

The flick follows an elder antique trader who dredges up a 500-year-old artifact that invigorates him with youth but has the unfortunate side effect of turning him into a vampire. — Carlos De Loera, Los Angeles Times, 5 Oct. 2026

The idea of my dad’s enduring intense treatments and sickening side effects for the rest of his life was dreadful. — Trisha Thadani Kirsten Potter Tanya Pérez Devin Murphy, New York Times, 4 Oct. 2026

Many of the side effects that can come with GLP-1 use are linked to weight loss, not the medications themselves. — Korin Miller, Allure, 2 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