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

However, there were some negative side effects—some who were unable to complete the month without alcohol reported drinking more afterwards. —Rachael O'Connor, MSNBC Newsweek, 6 Dec. 2025

Gastrointestinal Distress One of the most common side effects of high-dose vitamin C is gastrointestinal (GI) discomfort. —Sarah Jividen, Verywell Health, 5 Dec. 2025

Experts are now able to analyze the side effect with more patients being comfortable reporting their use of cannabis. —Vanessa Etienne, PEOPLE, 4 Dec. 2025

While incontinence is not thought to be a super common side effect of egg freezing, research shows that incontinence is widely underreported and underdiagnosed. —Anonymous, SELF, 4 Dec. 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