Very Heavy Cannabis Users Still Showed Decision-Making Deficits and Brain Changes After 25 Days Abstinent

Eleven heavy marijuana users and 11 non-drug using controls were studied using PET H2O15 imaging during the Iowa Gambling Task. Marijuana users resided in an NIH/NIDA inpatient research unit for 25 days prior to testing to ensure abstinence. Users were subdivided into moderate (8-35 joints/week) and heavy (53-84 joints/week) groups for dose-response analysis.

The finding of a potential threshold effect is significant. Moderate marijuana users showed brain activity and task performance comparable to non-users after 25 days of abstinence, while very heavy users did not. This suggests the level of use matters considerably for whether lasting cognitive effects emerge.

The orbitofrontal cortex and dorsolateral prefrontal cortex are regions critical for weighing risks and rewards in decision-making. Reduced activity in these areas after extended abstinence suggests that very heavy cannabis use may produce lasting changes in the brain circuits involved in evaluating consequences, though whether these changes are permanent remains unknown.

Very small sample size (11 per group). The study cannot determine whether brain differences preceded cannabis use or resulted from it. The threshold for "moderate" use (8-35 joints/week) is still substantial. Only male participants were included based on the study description. The Iowa Gambling Task is one measure of decision-making and may not capture all relevant cognitive domains.