The abuse of methylenedioxymethamphetamine (MDMA), flunitrazepam, ketamine hydrochloride, and gamma-hydroxybutyrate (GHB) is discussed. Club drugs are chemical substances used recreationally in social settings. Use is increasingly frequent among young people, especially during all-night dance parties. All four agents have been classified as controlled substances. MDMA ("ecstasy") is available as a tablet, a capsule, and a powder; formulations may contain many adulterants. MDMA increases the release of neurotransmitters. The desired effects are euphoria, a feeling of intimacy, altered visual perception, enhanced libido, and increased energy. The most common adverse effects are agitation, anxiety, tachycardia, and hypertension. More serious adverse effects include arrhythmias, hyperthermia, and rhabdomyolysis. Flunitrazepam is a potent benzodiazepine. At higher doses, the drug can cause lack of muscle control and loss of consciousness. Other adverse effects are hypotension, dizziness, confusion, and occasional aggression. Ketamine is a dissociative anesthetic used primarily in veterinary practice. It may be injected, swallowed, snorted, or smoked. Like phencyclidine, ketamine interacts with the N-methyl-D-aspartate channel. Analgesic effects occur at lower doses and amnestic effects at higher doses. Cardiovascular and respiratory toxicity may occur, as well as confusion, hostility, and delirium. GHB, a naturally occurring fatty acid derivative of gamma-aminobutyric acid, was introduced as a dietary supplement. Increasing doses progressively produce amnesia, drowsiness, dizziness, euphoria, seizures, coma, and death. Flunitrazepam, ketamine, and GHB have been used to facilitate sexual assault. Supportive care is indicated for most cases of club drug intoxication. The increasing abuse of MDMA, flunitrazepam, ketamine hydrochloride, and GHB, particularly by young people in social settings such as clubs, should put health care professionals on guard to recognize and manage serious reactions.