LSD stands for lysergic acid diethylamide. It is a psychoactive substance, which means that it acts on the central nervous system. This results in a change in behavior and the way the user relates to the world around them. These effects occur because of the way LSD affects the action of a brain chemical called serotonin. This substance helps control mood, thinking, behavior, and the senses. As Anton Gomez-Escolar tells us in the Essential Guide to the Psychedelic Renaissance, LSD was originally used as a psychotomimetic.