Despite the fact that people have been smoking marijuana for thousands of years, we know very little about what it does to the brain. There's anecdotal evidence that it impairs cognitive function, but there's equally compelling anecdotal evidence that it does nothing damaging whatsoever. We need more clinical evidence, but because it's a Schedule 1 substance, it cannot be federally approved for clinical trials. Additionally, it's hard finding volunteers who aren't afraid they'll be prosecuted for admitting they smoked a Schedule 1 substance, although we probably know a few. The shortage of knowledge may soon change, however, as the National Institutes of Health launched a landmark study in September 2015. The goal of the study is to find out what substance abuse (including alcohol, marijuana, and nicotine) does to adolescent brain development.
One study found that heavy marijuana usage does affect memory in adolescents. According to a Northwestern Medicine® study, teens who smoke pot are more likely to perform worse on memory tasks. The study found that two years after they stopped smoking, memory-related structures in their brains appeared to shrink and collapse inward, possibly reflecting a decrease in neurons. The groups in the study started using marijuana daily between 16 to 17 years of age for about three years.
Another study by the University College London found that light to moderate marijuana usage does not affect IQ in adolescents. The study sampled 2,235 British teenagers between the ages 8 and 16 and found that "cannabis use by the age of 15 did not predict either lower teenage IQ scores or poorer educational performance. These findings therefore suggest that cannabis use at the modest levels used by this sample of teenagers is not by itself causally related to cognitive impairment."
A study lead by Duke University also found that there's no evidence that smoking pot decreases IQ during adolescence. Researchers looked at 789 pairs of adolescent twins over the course of ten years. One twin smoked marijuana daily for six months and the other did not, and no reported difference in IQ was found as a result. This study has been critisized, however, becuase it lacks details in the frequency and amount of marijuana which the one twin smoked daily.
Until more research is published, we're left with very little data. With advances in technology like neuroimaging, it may be a short time before we can actually see how marijuana use affects the human brain, but it all hinges on the DEA's classification of marijuana.