Cognitive deficits in abstinent MDMA users were reported in various domains. The findings regarding attention, working memory, and executive function deficits are inconclusive, and were criticized for being induced by the co-use of other drugs. Moreover, social cognition and social decision-making have rarely been investigated in chronic MDMA users but in animals, long-lasting depletions of oxytocin (OXT) have been reported after chronic MDMA treatment implicating a potential impact on social cognition and behavior. The present thesis aims to eliminate the issue of unreliable self-reported drug use by investigating social and non-social cognition and social decision-making in objectively verified pure and polydrug MDMA users. Additionally, plasma OXT levels of a subsample were investigated. Verified pure MDMA users showed large deficits in declarative memory, while MDMA users with a co-use of stimulants shoved additional and stronger impairments in working memory and executive functions. Main MDMA users showed higher cognitive empathy but did not differ from controls in emotional empathy. Moreover, MDMA users acted less self-serving. However, higher hair MDMA values went along with a decrease in cognitive empathy. OXT plasma levels did not significantly differ between the two groups. Overall, our results suggest that even pure chronic MDMA users display strong impairments in declarative memory and that the broad cognitive impairments in polydrug MDMA users are driven by stimulant co-use. Additionally, they suggest that people with high cognitive empathy might be more prone to use MDMA chronically, which likely leads to a decrease in cognitive empathy.