The Effects of Cannabis on the Brain
People have been smoking marijuana for thousands of years, but we know very little about what it does to the brain. There's anecdotal evidence that it impairs cognitive function, but there's equally convincing anecdotal evidence that it does nothing damaging whatsoever. More clinical evidence is needed, but because it's a Schedule 1 substance in the US, it cannot be federally approved for clinical trials. Plus, it's difficult finding volunteers who are willing to admit they smoked a Schedule 1 substance, although we probably know a couple of folks. 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 by Northwestern Medicine shows that heavy cannabis usage does affect memory in the developing brains of adolescents. According to the 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 decrease and collapse inward, 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 Duke University study 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 available, we're left with precious little data. With advances in technology like neuroimaging, it may not be long before we can actually see how cannabis use affects the human brain, but it all hinges on the DEA's classification of the plant.