The research team discovered that individuals who used cannabis long-term (for several years or more) and heavily (at least weekly, though a majority in their study used more than four times a week) exhibited impairments across several domains of cognition.
Long-term cannabis users’ IQs declined by 5.5 points on average from childhood, and there were deficits in learning and processing speed compared to people that did not use cannabis. The more frequently an individual used cannabis, the greater the resulting cognitive impairment, suggesting a potential causative link.
The impact of cannabis on cognitive impairment was greater than that of alcohol or tobacco use. Long-term cannabis users also had smaller hippocampi (the region of the brain responsible for learning and memory). Interestingly, individuals who used cannabis less than once a week with no history of developing dependence did not have cannabis-related cognitive deficits. This suggests there is a range of recreational use that may not lead to long-term cognitive issues.