A pair of studies have shown that marijuana’s effects on cognitive performance are minimal and easily reversed. A Harvard study in the October 2001 Archives of General Psychiatry compared heavy pot smokers, light pot smokers and non-tokers with a battery of neuropsychological tests.
Four cognitive tests were performed after zero, one, seven and 28 days of cannabis-abstinence. Current pot smokers showed no deficits on three of the four cognitive tests.
After 28 days of abstinence, six more neuropsychological tests were performed on all subjects. No significant differences were found between the different groups.
The only test which showed a significant difference between the groups was on memory of word lists, in which the cannabis-users initially scored lower. However, this difference was undetectable after 28 days of abstinence. Researchers concluded that they had “failed to find an association between cumulative lifetime use of cannabis and cognitive deterioration.”
A separate study from Columbia University, published in the November 2001 Neuropsychopharmacology, found that smoking marijuana had minimal effects on complex cognitive task performance, including reaction time, memory and mental calculation.
The study examined experienced pot smokers, testing them before and after they were given joints of varying potency. Although marijuana use increased the time participants required to complete several tasks, “it had no effect on accuracy on measures of cognitive flexibility, mental calculation, and reasoning,” researchers concluded.