 Can the recreational use of marijuana cause cognitive impairment? Yeah, that's kind of the whole point. People clearly do not use cannabis only for its harms. Like, what about boosting creativity? That's one of the reasons people smoke it. But you don't know until you put it to the test. They looked at divergent thinking, the ability to brainstorm creative solutions to problems. And at a dose people might typically use to get high, their creativity took a hit too. So it may just be an illusion. People think they're more creative when they're high, but it may not be the best strategy and even turn out to be counterproductive. For a few hours after smoking one's learning, memory and attention may also be impaired. But the question is, does it cause any lasting problems? In other words, is cannabis neurotoxic for a healthy brain? Researchers have found that cannabis users have a significantly smaller hippocampus, the memory center in the brain, compared to non-users. Yeah, but a snapshot in time study can never prove cause and effect. What you have to do is follow people over time, only then can you see which came first. And what they found is both. There are pre-existing structural abnormalities in parts of the brain that control inhibitions and decision making that may make someone more likely to take up the drug, but the shrunken hippocampus does seem a consequence of chronic cannabis exposure. Okay, but is it permanent? There was a famous study published about pre-GPS London taxi drivers who spent literally years learning and memorizing how to navigate around the city, and they had hefty hippocampuses to prove it, correlating to the amount of time spent as a taxi driver, suggesting the structure of the brain is in constant flux. So if you stop using marijuana, does your hippocampus grow back to full size? Researchers tested users six months after quitting and still found shrinkage, but what about years later? We didn't know until now. Yeah, hippocampal volume is reduced in long-term cannabis users, but this atrophy can be restored following prolonged abstinence. Even after 15 years of use, 29 months after quitting, the size of their hippocampus appeared to bounce back, and the same with cognitive impairments gone within a month or two after stopping, unless you started regularly using as a teen. Those with the most persistent cannabis you starting as an adolescent may end up losing up to 8 IQ points significantly more than if they started as an adult, and even if they then quit starting that young appears to cause permanent brain damage. But to get that lasting damage may require both adolescent onset and almost two decades of persistent use. It sounds like if you start using as an adult, though there don't seem to be any irreversible neurological problems, unless perhaps you smoke like 16 joints a day. In 2017, a study was published on extreme chronic and heavy cannabis use, and their poor brains really did seem to go to pot, long-lasting brain dysfunction in more than half, and even long-lasting psychotic symptoms, hallucinations, delusions, and not just memory problems, but like difficulty drawing basic figures. But again, this was at 10 times the average daily dose in Colorado, for example, a total lifetime consumption of around 75,000 joints.