A study by doctors from the National Institute of Drug Abuse found that people who smoked marijuana had changes in the blood flow in their brains even after a month of not smoking. The marijuana users had PI (pulsatility index) values somewhat higher than people with chronic high blood pressure and diabetes, which suggests that marijuana use leads to abnormalities in the small blood vessels in the brain. These findings could explain in part the problems with thinking and remembering found in other studies of marijuana users.

According to two studies, marijuana use narrows arteries in the brain, similar to patients with high blood pressure and dementia, and may explain why memory tests are difficult for marijuana users. In addition, chronic consumers of cannabis lose molecules called CB1 receptors in the brain‘s arteries, leading to blood flow problems in the brain which can cause memory loss, attention deficits, and impaired learning ability.

Source: drugabuse.gov