A side effect was an unintended condition that developed as a result of ingesting a chemicalsubstance or medication or receiving medicaltreatment.
In 1986, Spock recalled that nuclear fission reactors were used in the 20th century, causing toxic side effects. (Star Trek IV: The Voyage Home)
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. (TOS: "The Cloud Minders")
According to Dr.Katherine Pulaski in 2365, reliving memories was a natural side effect of neural stimulation. (TNG: "Shades of Gray")
