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

Their blood vessel function was roughly half compared to those who did not use cannabis, with the side effect linked to a higher risk of heart attack, high blood pressure and other cardiovascular conditions, the researchers noted. —Ashley J. Dimella, FOXNews.com, 10 Jan. 2026

In those same studies, about 7% of patients or less stopped treatment due to side effects from the Zepbound and Wegovy injections. —Annika Kim Constantino, CNBC, 10 Jan. 2026

People imagine everyone on a GLP-1 is starving themselves, wasting away, or too sick from side effects to eat…and that’s simply not true. —Self Staff, SELF, 9 Jan. 2026

But these drugs can have serious side effects and require careful study before they can be repurposed for addiction. —Habibeh Khoshbouei, The Conversation, 9 Jan. 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