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 study also found that people who knew these long-term cannabis users well observed that they had developed memory and attention problems. The above findings persisted even when the study authors controlled for factors such as dependence on other drugs, childhood socioeconomic status, or baseline childhood intelligence.
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). https://www.health.harvard.edu/blog/cognitive-effects-of-long-term-cannabis-use-in-midlife-202206142760