Use of the illicit recreational drug 3,4-methylenedioxymethamphetamine (MDMA, Ecstasy) is one of the fastest growing drug abuse problems. MDMA has serious and potentially fatal toxic effects with increasing numbers of MDMA abuse related emergency room visits and deaths. MDMA undergoes extensive multiple cytochrome P450 (CYP) isoform-mediated metabolic elimination that generates active metabolites. Moreover, MDMA has a narrow recreational window with little margin between recreational and toxic blood levels. Hence drug-drug interactions could be a contributing factor to MDMA toxicity. Users often co-administer fluoxetine, an antidepressant and potent inhibitor of CYP2D6 with MDMA, in order to alleviate post-use depression. We investigated the possible pharmacokinetic drug interaction between MDMA and fluoxetine. Co-administration of MDMA with fluoxetine resulted in a significant increase in the exposure (1.5 fold) and prolonged the elimination half-life (2-4 fold) of MDMA and its active metabolite 3,4-methylenedioxyamphetamine (MDA) in the blood and the brain of rats. The persistent-elevated levels of MDMA and MDA suggested an enhanced risk of MDMA toxic effects upon combining MDMA with fluoxetine. However, we observed an attenuation of the MDMA pharmacodynamic effects (adrenergic-noradrenergic stimulation responsible for cardiovascular and temperature toxic effects of MDMA) upon co-administration with fluoxetine in rats. The observed paradox was due to an overwhelming antagonism of MDMA effects at its molecular site of action by fluoxetine. Over recent years, membrane transporters have been realized as important determinants of pharmacokinetics, drug response and drug interactions. However, little is known about interaction of MDMA with membrane transporters. We evaluated substrate activity of MDMA for the drug efflux transporter P-glycoprotein and the drug influx organic cation transporters using various in vitro (bidirectional transport and uptake studies) and in vivo (knock out mice) models. MDMA did not display substrate activity for these evaluated transporters. Furthermore, transport studies indicated that MDMA is a highly permeable agent and hence drug transporters will not play a physiologically relevant role in determining drug interactions of MDMA. Liver damage is one of the most serious and life threatening tissue-specific toxic effect of MDMA. However, the underlying mechanism is not known. Our mechanistic studies using a proteomics approach revealed oxidative modification and inactivation of mitochondrial proteins involved in energy metabolism as a mechanism for MDMA-mediated liver damage. In addition, these studies revealed that MDMA inactivated mitochondrial aldehyde dehydrogenase, an enzyme that catalyzes the elimination of acetaldehyde, a toxic metabolite of ethanol. Upon co-administration of ethanol with MDMA, a trend towards an increase in blood acetaldehyde levels was observed in rats, which suggests a clinically relevant and potentially toxic drug interaction between MDMA and ethanol.