Cannabis affects blood flow in the brain long after the last puff. US researchers say marijuana users have faster blood flow in their brains, even after a month of not smoking.
The findings suggest they have narrowed arteries, similar to patients with high blood pressure and dementia, and could helpto explain reports that heavy marijuana users have trouble on memory tests, said the researchers at the National Institute on Drug Abuse in Baltimore.
Ronald Herning and Jean Lud Cadet tested 54 marijuana users - who smoked anywhere between two and 350 joints a week - and 18 non-smokers.
They measured blood flow in the volunteers' brains at the beginning of the study and a month later, after everyone agreed to abstain from marijuana for the four weeks.
The smokers had faster blood flow, both at the start and after a month of abstinence, Herning and Cadet reported in Neurology.
The smokers also had a higher pulse index score, or PI, which measures the amount of resistance to blood flow. The researchers believe the higher PI is caused by narrower blood vessels.
"The marijuana users had PI values that were somewhat higher than those of people with chronic high blood pressure and diabetes," Herning said in a statement.
"However, their values were lower than those of people with dementia. This suggests marijuana use leads to abnormalities in the small blood vessels in the brain."
They found that blood flow improved in people who smoked up to 70 joints a week - moderate users - after a month of avoiding cannabis.
Heavy users, who smoked as many as 350 joints a week, saw no change in blood flow even after a month of abstaining, the researchers said.
Researchers at Montreal's McGill University have reported that chronic consumers of cannabis 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.
