The relative contributions of ecstasy and cannabis to cognitive impairment

Rationale: (+/-)-3,4-methylenedioxymethamphetamine (MDMA; 'ecstasy'), a commonly used recreational drug, has typically been found to be related to poor cognitive function in humans. However, cannabis consumption may not have been adequately controlled for in these studies. Objective: The present study was designed to further elucidate the relation between MDMA and cannabis in cognitive impairment. Methods: Subjects who had used neither MDMA nor cannabis (controls; n=31), cannabis but not MDMA (cannabis users; n=18) and both MDMA and cannabis (MDMA/cannabis users; n=11) were compared on a battery of neuropsychological tests. Results: The cannabis and MDMA/cannabis groups did not differ on any of the tests, whereas the combined cannabis and MDMA/cannabis groups performed more poorly than controls on tests of memory, learning, word fluency, speed of processing and manual dexterity. Further, apart from speed of processing where higher MDMA consumption predicted slower processing, covariate analysis revealed that the deficits were more closely related to cannabis than MDMA usage. Conclusion: The results suggest that cannabis is an important confound in studies of MDMA-related cognitive impairment, and that previously reported cognitive impairment in MDMA users may have been caused by coincident cannabis use.