Acute intoxication with cannabis clearly produces cognitive impairment, but it is less clear how long cognitive deficits persist after an individual stops regular cannabis use. Numerous methodologic difficulties confront investigators in the field attempting to assess the residual neuropsychologic effects of cannabis among heavy users, and these must be understood to properly evaluate available studies. At present, it appears safe to conclude that deficits in attention and memory persist for at least several days after discontinuing regular heavy cannabis use. Some of these deficits may be caused or exacerbated by withdrawal effects from the abrupt discontinuation of cannabis; these effects typically peak after 3 to 7 days of abstinence. It is less clear, however, whether heavy cannabis use can cause neurotoxicity that persists long after discontinuation of use. It seems likely that such long-term effects, if they exist, are subtle and not clinically disabling--at least in the majority of cases.