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

Drinking too much can cause side effects, including stomach irritation, slower digestion, acid reflux, tooth enamel wear, and skin irritation. —Merve Ceylan, Health, 22 Nov. 2025

Many companies are developing drugs to counteract side effects from GLP-1s, which include muscle loss and macular degeneration—an eye disease that may cause vision loss. —Angelica Ang, Fortune, 21 Nov. 2025

Some people may experience uncomfortable gastrointestinal side effects when taking creatine and magnesium, such as nausea and bloating. —Stephanie Brown, Verywell Health, 21 Nov. 2025

But people who take these drugs often experience unpleasant side effects. —NPR, 21 Nov. 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