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

Prasad focused on myocarditis, or heart inflammation, a very rare side effect after administration of the mRNA vaccines. —Ben Tinker, CNN Money, 12 Dec. 2025

Steering clear of foods that cause digestive side effects. —Caroline Tien, SELF, 12 Dec. 2025

But health authorities worldwide, including at the Centers for Disease Control and Prevention, have emphasized that the benefits of vaccination in preventing severe Covid and even myocarditis from the infection itself far outweigh the small risk of the side effects posed by the shot. —Annika Kim Constantino, CNBC, 12 Dec. 2025

He was hospitalized earlier this year after experiencing side effects amid his care. —Anna Kaufman, USA Today, 12 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