The stereotype of marijuana users who seem a few cards short of a full deck may not be a myth, dude. Scientists have shown that people who use the drug have problems with memory and on tests that measure reasoning ability, and that part of the problem could be that marijuana affects blood flow to the brain. No one is sure, however, whether blood will flow normally again once a person quits, or whether the drug damages a person's brain more permanently. Scientists from the National Institute on Drug Abuse tried to figure that part out.

What the researchers wanted to know: What happens to the blood flow in a person's brain after they have stopped using marijuana?

What they did: The researchers recruited 54 marijuana users, grouped into light users, who smoked the equivalent of 11 joints a week, moderate users, who smoked about 44 joints a week, and heavy users, who smoked the equivalent of 131 joints a week on average. All of the patients checked into an inpatient research clinic for 30 days. The researchers used a sonographic scanning machine to measure blood flow in their brains within 72 hours after they had been admitted to the clinic, and again after they had been in the clinic (and sober) for at least 28 days. They compared the measurements of the marijuana users against a control group that did not use the drug.

What they found: The marijuana users had changes in the blood flow to their brains that remained in heavy users even after a month of abstinence. In particular, they had a higher cerebral blood flow velocity and higher pulsatility index, meaning that there was more resistance to blood flowing through the brain. These changes could partially explain why marijuana users often score lower on cognitive tests than controls do. The researchers compared the pulsatility index of these patients with others: The marijuana users have higher (worse) numbers than heart disease or diabetes patients but not as high as patients with dementia. After a month without marijuana, the light and moderate users improved, though the heavy users did not.

What it means to you: You don't have to look very far—be it in the movies, on the radio, or on television—to see someone get high and act stupid. The cause for that, according to these researchers, could be changes in the blood flow to a user's brain, leading to mental declines. These changes can be reversed in some patients over time, according to this study, but may take longer or even be permanent in heavy users of marijuana.

Caveats: Four people in the study also smoked cigarettes, which could also lead to constriction of the blood vessels of the brain. The researchers attempted to control for this complication by testing the smokers at least 20 minutes after their last cigarette, but it is hard to separate the effects. In addition, other drugs that the subjects had tried could have influenced their cerebral blood vessels, though in a questionnaire, the marijuana users reported not having used other substances.

Find out more: The National Institute on Drug Abuse has a Web page that discusses the health effects of marijuana as well as its prevalence.

The Marijuana Anonymous organization is a support group (akin to Alcoholics Anonymous) for people who want to quit using the drug.

Read the article: Herning, R.I. et al. "Cerebrovascular Perfusion in Marijuana Users During a Month of Monitored Abstinence." Neurology. Feb. 8, 2005, Vol. 64, No. 6, pp. 488-493.

Abstract online: www.neurology.org