Cognitive dysfunction — The association between chronic marijuana exposure and cognitive dysfunction has been extensively studied but with varying results. A syndrome formerly known as the “amotivational syndrome,” now called the “chronic cannabis syndrome,” has been described in which chronic heavy users with cognitive impairment have a reduced ability to establish or attain goals in life, resulting in jobs that require less cognitive challenge or technological acuity.

A biologic basis for chronic cannabis syndrome was suggested by a study of cerebral blood perfusion, comparing intracranial doppler sonography between chronic marijuana users (n = 54) and nonusers (n = 18). Subjects were studied over 30 days of abstinence in an inpatient research unit; chronic marijuana use resulted in increased cerebrovascular resistance in heavy users, which persisted over a 30 day abstinence period.

In a meta-analysis that included 13 studies, long-term marijuana use did not result in deficits in seven of eight neuropsychological ability areas tested; there was a small but significant decrement in the area of learning new information. Studies published after the meta-analysis have found conflicting results:

 
• In one study, cognitive deficits that were apparent at days zero, one, and seven after last marijuana use were reversible and not related to cumulative lifetime use.  
• A second study compared cognitive function in longer term and shorter term marijuana users at a median of 17 hours after last reported marijuana use. Deficits in memory, attention, learning and retrieval function were significantly greater among the longer-term marijuana users.  
• Longer term (LT) marijuana users (four or more joints per week for a minimum of 10 years), tested after a minimum of 24 hours abstinence, had impaired verbal memory skills compared to shorter term (ST) users and controls; both LT and ST users showed inferior performance on psychomotor speed, attention, and executive functions compared with controls.

Most of these studies used duration of marijuana use as the patient variable in determining cognitive outcomes. In a study that relied instead on the amount of marijuana smoked (average joints per day), 22 marijuana users (mean age 22 years) were divided into groups of light use (2 to 14 joints per week), middle use (19 to 70 joints per week), and heavy use (78 to 117 joints per week). After 28 days of inpatient abstinence, a dose response effect was found for tests of verbal memory, visual learning and memory, executive function, psychomotor speed, and manual dexterity. These effects are biologically plausible since the affected functions require use of frontal, cerebellar, and hippocampal brain regions, all of which contain cannabinoid receptors. Very heavy use of marijuana is associated with persistent decrements in neurocognitive performance even after 28 days of abstinence. However, despite the adverse effects of heavy marijuana use, performance on the majority of cognitive tests remained in the normal range. It is not clear whether continued heavy use would result in more significant declines.

In summary, the evidence is fairly consistent that marijuana use results in cognitive deficits that persist for at least hours, and likely days after acute intoxication. Whether these deficits persist in the long term and the effects of persistent heavy use have not been settled.