Club drugs, which are sometimes called "party drugs," are a disparate group of psychoactive substances that include methylenedioxymethamphetamine (MDMA), ketamine, and gamma-hydroxybutyrate (GHB), among others. These drugs differ in terms of administration, mechanism of action, and effect. What unites them, and why they are grouped together here, is that in many cases they are used by groups of younger people in social settings, often at raves or parties. They are often taken in various combinations. Polysubstance use might be considered the norm at such events, with more than 80% of participants using more than one illicit substance (Boys et al. 1997; Winstock et al. 2001).