side effect

Or side-ef·fect

noun

1. any effect of a drug, chemical, or other medicine that is in addition to its intended effect, especially an effect that is harmful or unpleasant.

2. any accompanying or consequential and usually detrimental effect.

   the side effects of air pollution.

side effect

noun

1. any unwanted nontherapeutic effect caused by a drug Compare aftereffect

2. any secondary effect, esp an undesirable one

Word History and Origins

Origin of side effect¹

First recorded in 1880–85

Example Sentences

Early reports flagged a type of heart swelling called myocarditis as a rare side effect of the mRNA vaccine, particularly for young men ages 18 to 25 after a booster dose.

From Salon

Neither are available on the NHS and both can cause unwanted side effects.

From BBC

This much is clear: The president’s cruel, fascist theater isn’t a reaction or side effect.

From Salon

But as Kershaw got older, his routine also came with increasingly damaging physical side effects.

From Los Angeles Times

Another study indicated that intermittent fasting may produce side effects such as weakness, hunger, dehydration, headaches and difficulty concentrating.

From BBC

Related Words

• aftereffect 
• reaction