Side effect
A side effect was an unintended condition that developed as a result of ingesting a chemical substance or medication or receiving medical treatment.

In 1986, Spock recalled that nuclear fission reactors were used in the 20th century, causing toxic side effects. ()

In 2269, Spock was surprised to learn that zenite was dangerous to the Troglytes, as no side effects had ever been noted. Leonard McCoy explained that processed zenite was safe, but its raw form emitted an odorless, invisible gas which caused intelligence to decrease and emotional reactions to increase. ()

According to Doctor Katherine Pulaski in 2365, reliving memories was a natural side effect of neural stimulation. ()

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Medical conditions