Cannabis Has No Long-term Effect On Cognitive Performance

http://healthland.time.com/2011/07/19/study-marijuana-not-linked-with-long-term-cognitive-impairment/

The idea that “marijuana makes you dumb” has long been embodied in the stereotype of the slow, stupid stoner, seen in numerous Hollywood movies and TV comedies and going unquestioned by much of American culture. But a new study says no: the researchers followed nearly 2,000 young Australian adults for eight years and found that marijuana has little long-term effect on learning and memory— and any cognitive damage that does occur as a result of cannabis use is reversible.

When the results were at last tabulated, researchers found that there were large initial differences between the groups, with the current marijuana smokers performing worse on tests that required them to recall lists of words after various periods of time or remember numbers in the reverse order from the one in which they were presented.

However, when the investigators controlled for factors like education and gender, almost all of these differences disappeared.  The lower education levels of the pot smokers — and their greater likelihood of being male — had made it look like marijuana had significantly affected their intelligence.  In fact, men simply tend to do worse than women on tests of verbal intelligence, while women generally underperform on math tests. The relative weighting of the tests made the impact of pot  look worse than it was. They found that heavy users who had quit by the end of the study were no longer distinguishable on this measure from those who had never used.

T]he adverse impacts of cannabis use on cognitive functions either appear to be related to pre-existing factors or are reversible in this community cohort even after potentially extended periods of use.

Turns out generally stoners aren't stupid and any adverse effects can be reversed. Looks like they aren't putting themselves at risk of anything.