Marijuana smokers develop changes in the flow of blood to the brain, which may explain why smoking marijuana can impair thinking and memory, according to new study findings released Monday.

Investigators from the National Institutes of Health in Baltimore, Maryland, found marijuana users’ blood vessels showed a higher resistance to blood flow, a sign blood was having a harder time getting through tiny blood vessels in the brain.

And heavy smokers - who reported smoking an average of 131 joints per week - showed the same increases in the resistance to blood flow one month after quitting cold turkey, according to a report in the journal Neurology.

These changes may occur if marijuana causes the blood vessels to constrict, or if the vessels become clogged with plaques, study author Dr. Ronald I. Herning suggested.

Over time, marijuana users may develop problems if these changes prevent the brain from getting the blood it needs. “Your brain needs blood and oxygen for the neurons to fire, and do their job,” Herning told Reuters Health in an interview.

For instance, people may begin to have trouble thinking and remembering. There may be more serious consequences, as well, the researcher added. “With these changes, there could be a risk of stroke, actually,” he said.

To investigate how marijuana may affect blood flow to the brain, Herning and his team used ultrasound to measure blood flow in the blood vessels feeding the brain in 54 marijuana users and 18 non-users. The researchers examined the blood vessels 3 days after all smokers had stopped using the drug, and 28 to 30 days after monitored abstinence.

Herning and his team noted significant changes in blood flow among marijuana users that were not seen in non-users. Moreover, one month after not smoking, heavy users still showed the same changes in blood flow, the investigators report.

The next step, Herning said, is to continue to follow study participants to determine if the changes are permanent in heavy users. “More research really needs to be done,” he said.

SOURCE: Neurology, February 8, 2005.