Lysergic acid diethylamide, or LSD, was a powerful psychotropic drug once used, as a recreational drug, on Earth. LSD was also experimented with by several government agencies on Earth for use as a form of mind control. In 1986, when James T. Kirk and his crew had travelled back in time, Kirk attempted to explain Spock's behaviour to Gillian Taylor, a native of the time, by explaining Spock had taken too much LSD (or LDS as he mistakenly referred to it) in the 1960s. (TOS movie: The Voyage Home)