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

Experts say testosterone therapy can also cause the following side effects. —Shiv Sudhakar, FOXNews.com, 4 Oct. 2025

This can cause side effects like anxiety, restlessness, irregular heart rate, irritability, and muscle twitches. —Carley Millhone, Health, 4 Oct. 2025

Part of this is taking each individual menopause side effect a woman experiences into consideration and treating them one by one. —Helen Carefoot, Flow Space, 2 Oct. 2025

Beyond the mental clarity and razor sharp focus, taking mushrooms like lion’s mane also comes without many of the potential side effects that are typically associated with stimulants like jitters, energy spikes, and mid-day crashes. —Brianna Peters, Vogue, 1 Oct. 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