Marijuana users have faster blood flow in their brains, even after a month of not smoking, US researchers reported today.
The findings suggest they have narrowed arteries, similar to patients with high blood pressure and dementia, and could help explain why heavy marijuana users appear to have trouble on memory tests, said researchers at the National Institute on Drug Abuse in Baltimore.
Mr Ronald Herning and Ms Jean Lud Cadet tested 54 marijuana users, who smoked anywhere between two and 350 joints a week, and 18 non-smokers.
They used Doppler sonograms to measure blood flow in 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, Mr Herning and Ms Cadet reported in the journal Neurology.
The smokers also had a higher pulsatility index score, or PI, which measures the amount of resistance to blood flow.
"The marijuana users had PI values that were somewhat higher than those of people with chronic high blood pressure and diabetes," Mr Herning said.
"However, their values were lower than those of people with dementia. This suggests that marijuana use leads to abnormalities in the small blood vessels in the brain."
The researchers found that blood flow improved in people who smoked up to 70 marijuana cigarettes a week - people they defined as moderate users - after a month of avoiding cannabis.
Heavy users, who smoked up to 350 joints a week, saw no change in blood flow even after a month.