"Marijuana smoke contains a lot of the same chemicals in tobacco smoke and so it would not be at all surprising if it had some of the same ill effects, like cardiovascular disease and lung cancer. But when people look for these effects, they can’t find any increase in mortality among marijuana smokers. I predict that larger studies will one day pick something up"
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"As far as I can tell the finding still seems legit, and marijuana use does decrease IQ."
This 2014 article makes some pretty bold predictions and claims.