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

Each patch contains a relatively low level of each ingredient, which could technically reduce the likelihood of side effects like nausea and rashes. —Helena Kudiabor, SELF, 13 Mar. 2026

Other side effects may include lower coordination, difficulty waking up, and tiredness the next day. —Kirstyn Hill, Health, 13 Mar. 2026

My derm recommended using vitamin C products (in conjunction with SPF) in the morning to not only help with the side effects, but to enhance my complexion (think brighter, younger, healthier-looking skin). —Gina Vaynshteyn, StyleCaster, 13 Mar. 2026

This is just one new side effect from a push by company executives to make workers use AI more. —Allison Morrow, CNN Money, 13 Mar. 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