MDMA (3,4-methylenedioxymethamphetamine, ecstasy) is a widely misused psychostimulant drug abused among large segments of the young population. Pharmacologically it displays effects related to amphetamine-type drugs and a set of distinctive effects (closeness to others, facilitation to interpersonal relationship, and empathy) that have been named by some authors “entactogen” properties. MDMA is a potent releaser and/or reuptake inhibitor of presynaptic serotonin (5-HT), dopamine (DA), and norepinephrine (NE). These actions result from the interaction of MDMA with the membrane transporters involved in neurotransmitter reuptake and vesicular storage systems. The most frequent effects after MDMA/ecstasy administration are euphoria, well-being, happiness, stimulation, increased energy, extroversion, feeling close to others, increased empathy, increased sociability, enhanced mood, mild perceptual disturbances, changed perception of colors and sounds, somatic symptoms related to its cardiovascular and autonomic effects (blood pressure and heart rate increase, mydriasis), and moderate derealization but not hallucinations. Acute toxic effects are related to its pharmacologic actions. The serotonin syndrome (increased muscle rigidity, hyperreflexia, and hyperthermia), among others, is characteristic of acute toxicity episodes. MDMA metabolism is rather complex and includes 2 main metabolic pathways: (1) O-demethylenation followed by catechol-O-methyltransferase (COMT)-catalyzed methylation and/or glucuronide/sulfate conjugation; and (2) N-dealkylation, deamination, and oxidation to the corresponding benzoic acid derivatives conjugated with glycine. The fact that the polymorphic enzyme CYP2D6 partially regulates the O-demethylenation pathway prompted some expectations that subjects displaying the poor metabolizer phenotype may be at higher risk of acute toxicity episodes. In this metabolic pathway a mechanism-based inhibition of the enzyme operates because the formation of an enzyme–metabolite complex that renders all subjects, independently of genotype, phenotypically poor metabolizers after the administration of 2 consecutive doses. Therefore, the impact of CYP2D6 pharmacogenetics on acute toxicity is limited. One of the interesting features of MDMA metabolism is its potential involvement in the development of mid- to long-term neurotoxic effects as a result of progressive neurodegeneration of the serotonergic neurotransmission system.