MDMA is a chiral molecule, possessing two enantiomers, S(+)-MDMA and R(-)-MDMA, with S(+)-MDMA being more potent than R(-)-MDMA [48, 49]. All research in humans to date and the majority of nonclinical studies have used racemic MDMA, or an admixture containing equal amounts of both enantiomers. Studies of drug discrimination in rodents [50, 51] and self- administration in primates [52] suggest that not only do the enantiomers produce different effects, but that there may be some synergy between the two. It seems that R(-)-MDMA may have hallucinogen-like effects, compared to S(+)-MDMA, which exhibits psychomotor stimulant-like effects. According to an in vivo microdialysis study, S(+)-MDMA may be associated with greater dopamine release in specific brain areas [53]. A recent study in monkeys found that S(+)-MDMA, but not R(-)-MDMA, significantly increased extracellular dopamine levels in the dorsal striatum, whereas S(+)-MDMA significantly increased serotonin levels [54]. MDMA available for human use is racemic, containing roughly equal amounts of both enantiomers. Any differential effects of the enantiomers remain untested in humans.