3,4-Methylenedioxymethamphetamine (MDMA; 'ecstasy') is an increasingly popular recreational drug in the US, Western Europe and Australia. In animals, including nonhuman primates, MDMA is known to damage brain serotonin (5-hydroxytryptamine; 5-HT) neurons. It is not known whether MDMA damages serotonin neurons in the human brain but there is some indication that it may. Although the large majority of individuals who have used MDMA recreationally do not develop acute complications, as the popularity of MDMA has increased, so have reports of adverse nonpsychiatric and psychiatric consequences associated with use of the drug. Further, since manifestations of MDMA-induced serotonin injury might only become apparent with age, or under periods of stress, it is possible that some individuals with no apparent abnormalities might develop complications over time.