A long-term study published in August reported that adolescents who are regular users of marijuana have lower IQ's than kids who don't. The study said that on average, the teen pot-heads scored eight points lower on IQ tests than their non-smoking counterparts. While eight points doesn't sound like much, the study reports, "a drop from 100 to 92 represents a move from the 50th to the 29th percentile."
While all that sounds convincing, a new study just came out saying that it's all bullsh*t. The second study is saying that the IQ differences could be due to their socioeconomic status, not their pot use. Other studies from the same group of kids have found "that early-onset cannabis use is more common for those with poor self-control, prior conduct problems, and high scores on risk factors correlated with a low family SES," according to Ole Rogeberg, who authored the new analysis.
Basically, kids that come from lower-class families are more likely to toke up and less likely to be put into intellectually challenging environments, which is an important factor for developing a child's brain.
Another flaw in the original study is that it is only based off a small population of kids.
Overall, we've learned not to trust a single study. Hell, even this second study could be wrong. There's always going to be researchers that find an excuse for why pot is bad and others who will say it's just fine.
Either way, both studies agree that cannabis has no effects on the brain of adults.