The effects of marijuana in the brain may linger long after the last joint goes out.

A new study shows that blood flow to the brain in people who smoked marijuana (search) remained altered up to a month after they last smoked pot.

Researchers say the findings may help explain the problems with memory and thinking found in previous studies of chronic marijuana users.

Marijuana's Effects on the Brain

In the study, which appears in the Feb. 8 issue of Neurology, researchers studied the blood flow in brain arteries of 54 marijuana users and 18 nonusers.

The marijuana users volunteered to participate in an inpatient program and abstained from marijuana use for a month.

Blood flow in the brain was analyzed at the beginning of the study and at the end of the month for the marijuana users.

Researchers found blood flow was significantly higher in marijuana users than in nonusers, both at the beginning and at the end of the study.

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However, the marijuana users also had higher scores on the pulsatility index (search) (PI), which is a measure of resistance to blood flow.

Researchers say the level of resistance to blood flow among light and moderate marijuana users improved over the course of the abstinence month. But there was no improvement among heavy marijuana users.

This resistance is thought to be caused by the narrowing of blood vessels that happens when the body's own ability to regulate the circulatory system becomes impaired.

"The marijuana users had PI values that were somewhat higher than those of people with chronic high blood pressure and diabetes," says researcher Ronald Herning, PhD, of the National Institute on Drug Abuse (search) in Baltimore, Md., in a news release. "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, because similar PI values have been seen in other diseases that affect the small blood vessels."

Light marijuana users smoked two to 15 joints per week, moderate users smoked 17 to 70 joints per week, and heavy users smoked 78 to 350 joints per week.

By Jennifer Warner, reviewed by Brunilda Nazario, MD

SOURCES: Herning, R. Neurology, Feb. 8, 2005; vol 64: pp 488-493. News release, American Academy of Neurology.