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, more uncommon but serious side effects, like acute pancreatitis (inflammation of the pancreas) and gallbladder issues, need immediate medical attention. —Amaris Encinas, USA Today, 21 Feb. 2026

Next in his sights is a remaking of the National Vaccine Injury Compensation Program, the fund that pays people who suffer from a rare side effect from a shot. —Lisa Jarvis, Mercury News, 21 Feb. 2026

Regulators may consider some broad factors—from side effects to drug administration—the experts say. —Lauren J. Young, Scientific American, 20 Feb. 2026

Consider keeping a journal for about a month to track your overall well-being, changes in symptoms, and any side effects. —Heidi Moawad, Verywell Health, 20 Feb. 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