Marijuana has a long-term effect on blood flow to the brain, potentially increasing the risk of memory damage and stroke, research finds.
Marijauna appears to constrict blood vessels
The National Institute on Drug Abuse in Baltimore found users have faster blood flow in their brains - even after a month of not smoking.
The findings suggest marijauna use narrows blood vessels, in way similar to that found in heart disease.
Details of the study are published in the journal Neurology.
The researchers tested 54 marijuana users, who smoked between two and 350 joints a week, and 18 non-smokers.
They used sophisticated ultrasound technology to measure blood flow in volunteers' brains at the beginning of the study and after a month of abstinence.
Marijauna smokers had a faster blood flow, both at the start of the study, and after they had refrained from their habit for four weeks.
The smokers also had a higher pulsatility index (PI) score. This is a measure of the resistance to blood flow.
The researchers believe the higher PI is caused by narrower blood vessels.
Researcher Dr Ronald Herning said: "The marijuana users had PI values that were somewhat higher than those of people with chronic high blood pressure and diabetes.
"This suggests that marijuana use leads to abnormalities in the small blood vessels in the brain."
The research also found that after a month of not smoking, people who had been moderate users - smoking up to 70 marijuana cigarettes a week - showed signs of improved blood flow.
However, a month of abstinence had no positive impact on blood flow among heavy users, who smoked up to 350 joints a week.
Dr Herning told the BBC News website that if blood supply to the brain was reduced, then its cells would be starved of oxygen, and unable to function at full capacity.
"In the long-term one might see cognitive difficulties, such as problems with memory and thinking," he said.
He said it was also possible that using the drug may increase the risk of a stroke, which is caused by damage to the blood vessels in the brain.
There are case reports of strokes among young marijauna users.
"My advice would be to abstain from using the drug," Dr Herning said.
Research by a team at McGill University in Montreal has found that long-term cannabis users lose molecules called CB1 receptors in the brain's arteries.
This reduces blood flow to the brain, causing attention deficits, memory loss, and impaired learning ability.
Professor Peter Weissberg, medical director of the British Heart Foundation, said: "This paper adds to the growing body of evidence that marijuana has important and persistent physiological effects on blood flow to the brain that may have longterm consequences.
"This, coupled with the likelihood that marijuana uses are often also tobacco smokers, points to potential longterm risks for frequent users."