The new study of cognitive changes caused by heavy marijuana use has found no lasting effects 28 days after quitting. Following a month of abstinence, men and women who smoked pot at least 5,000 times in their lives performed just as well on psychological tests as people who used pot sparingly or not at all, according to a report in the latest edition of the Archives of General Psychiatry.
That's the good news. The bad news, not included in the study, is that most heavy users admit that pot has had a negative effect on their physical and mental health as well their functioning on the job and socially.
"If there's one thing I've learned from studying marijuana for more than a decade, it's that proponents and opponents of the drug will put opposite spins on these findings," says Harrison Pope, a Harvard professor of psychiatry and leader of the research. "One day I'll get a letter that will say, 'we are shocked that you are so irresponsible as to publish a report that claims marijuana is almost harmless. That's a terrible disservice to our children.' The next day, I'll get a letter complaining that I'm 'irresponsible for implying there's something wrong with smoking marijuana. You have set back the legalization (of marijuana) movement by 20 years.'
Harvard Gazette: Study: Intelligence, cognition unaffected by heavy marijuana use
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