A recent study (Addiction journal) completed after 8 years of data concludes that there are no long term cognitive impairments from marijuana use. The study followed over 2000 Australians aged 20-24 for eight years, and subjected them to a number of tests that measured their cognitive performance. The study group self reported their level of cannabis use from no use to regular heavy use, including whether or not they had stopped using.
Over the eight year period, the subjects were tested three times and were interviewed about their use of marijuana. The data showed that current marijuana smokers performed worse on the tests of short term memory, such a memorizing numbers or recalling lists of words. However once researchers normalized the results to account for education and other factors, they realized that the initial results made the impact of marijuana look worse than it actually was.
Education was one factor that influenced the results. The study found that people with lower education levels were more likely to smoke marijuana. The other major factor was gender. Marijuana smokers tent to be male, and makes perform worse than women on tests of verbal intelligence.
The study also determined whether the slight cognitive impairment that was demonstrated by heavy users was permanent. They found that heavy users who had quit by the end of the study were no longer distinguishable from those who never used cannabis at all.
The study concluded that quitting marijuana use is associated with improvement in capacity to recall information that has just been learned. No other measures of cognitive performance were found that were related to marijuana use.
Study: Marijuana Not Linked with Long Term Cognitive Impairment