This article summarizes the short-term physiological toxicity and the adverse behavioral effects of four substances (GHB, ketamine, MDMA, and Rohypnol) that have been used at latenight dance clubs. The two primary data sources were case studies of human fatalities and experimental studies with laboratory animals. A safety ratio was calculated for each substance based on its estimated lethal dose and its customary recreational dose. GHB (gamma-hydroxybutyrate) appears to be the most physiologically toxic; Rohypnol (flunitrazepam) appears to be the least physiologically toxic. The single most risk-producing behavior of club drug users is combining psychoactive substances, usually involving alcohol. Hazardous drug-use sequelae such as accidents, aggressive behavior, and addiction were not factored into the safety ratio estimates.