From Daniel McGuire:

Re: Brain abnormalities seen in heavy pot smokers (June 2): Do a quick google search for “THC Cancer” and see what you find.

The 15 person study that showed smaller brain structures on average for 15 people who chose to smoke an average of 5 joints a day for a lot of years hardly has bearing on the great majority of pot smokers. In fact, if 5 joints a day is a stupid amount, then maybe people with smaller brain structures in the first place are the ones who are going to be more likely to smoke that amount. This study shows no causal relationship whatsoever between pot and the smaller brain structures.

The anti-cancer THC research on the other hand seems solid and is being repeated in many universities around the world.