A 2001 study showed that a lifetime of marijuana smoking was not associated with deficits in general intellectual function, abstraction ability, sustained attention, verbal fluency or ability to learn and recall new verbal and visuospatial
information after one month of abstinence. This was a controlled study of 180 volunteers including 63 heavy marijuana users (who had smoked cannabis at least 5,000 times in their lives), all smoking daily at the start of the study; 45 former heavy users who had smoked no more than 12 times in the previous three months; and 72 control subjects who had smoked no more than 50 times in their lives. Recall of word lists was reduced in heavy users who smoked daily at entry into the study only during the first week of abstinence. [Arch Gen Psychiatry 2001;58(10): pp.909-15]

Any negative effect on IQ as a result of marijuana use may be temporary, as seen in the following study. Current smoking of five or more marijuana joints per week (assessed by self-reporting and urinalysis) was associated with reduced global IQ scores in a study of 70 subjects 17-20 years old. A negative effect was not observed among subjects who had previously been heavy users but were no longer using marijuana. [CMAJ 2002;166(7): pp.887-93]

Marijuana smoking was, however, associated with an increased risk of depression in a study of 1,920 people with no symptoms of depression at the onset of the study who were reassessed approximately 15 years after baseline data was
collected. [Am J Psychiatry 2001;158(12): pp.2033-37]

Withdrawal symptoms (marijuana craving, decreased appetite, sleep difficulty and weight loss) did occur in an assessment of 12 marijuana smokers on 16 consecutive days during which they smoked marijuana as usual (days 1-5), abstained from smoking marijuana (days 6-8), returned to smoking marijuana (days 9-13), and again abstained from smoking marijuana (days 14-16). Aggression, anger, irritability, restlessness and strange dreams increased significantly only during one of the abstinence phases. [Arch Gen Psychiatry 2001;58(10): pp.917-24]