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Memory up in smoke with long marijuana use

UPI Consumer Health Correspondent

WASHINGTON, March 13 (UPI) -- Smoking marijuana long term could make your memory and other cognitive skills go -- quite literally -- to pot.

In a new study of long-term users people who smoked at least one marijuana cigarette a day for 10 years performed poorly on a range of standardized tests, including verbal fluency, verbal memory and learning, as compared to both those who had lit up for shorter amounts of time or did not use marijuana at all.

"It will help us understand that cannabis is not such an innocent drug," said lead author Dr. Lambros Messinis, a neurologist at the University Hospital of Patras in Patras, Greece.

His research will be published in the March 14 issue of Neurology, the scientific journal of the American Academy of Neurology.

Scientists understand the intoxication effects of cannabis, but the long-term impacts of the drug on the mind are not as well-documented. This prompted Messinis to address what he called a "significant absence" of studies on the subject.

Messinis and colleagues recruited 64 people from a drug-abuse treatment program in Athens and divided them into three groups: long-term frequency users, who had smoked for at least 10 years; short-term frequency users, who smoked for five to 10 years, and a control group of people who may have tried marijuana in the past, but sparingly. Frequent users ingested marijuana more than 20 times a month. The participants, aged 17 to 49, were tested to ensure they were not using other drugs at the time of the analysis.

The subjects also refrained from smoking marijuana for at least 24 hours in order to avoid the effects of intoxication altering the experiment.

Researchers then administered various cognitive tests to participants. The most striking deficits among the long-term users appeared in verbal learning: the subjects had difficulty in recalling previously learned words, for instance. Messinis also discovered the long-term users had trouble with executive functioning, or the ability to organize and coordinate simple tasks.

So what does marijuana do once it hits the brain? Messinis said no one knows for sure, and he did not want to speculate. However, the Web site of the National Institute on Drug Abuse suggests it's likely marijuana attaches to receptors in the brain, interfering with normal neurotransmission.

Sections of the brain with the most receptors include the cerebellum, the cerebral cortex, and limbic system: all areas responsible for thinking, problem solving, balance and memory, among others.

Yet some view the results with skepticism.

For instance, the data does not support the conclusion that heavy long-term use leads to permanent deficits, which is implied in the research, said Igor Grant, a professor of psychiatry and director of the Center for Medicinal Cannabis Research at the University of California in San Diego.

"It's not a surprise to anybody that people who are active users have subtle impairments," Grant said.

Grant authored a 2003 paper in the Journal of the International Neuropsychological Society, which reviewed many studies on the topic of long-term cannabis use and concluded that brain effects, if any, are minimal.

"I have no doubt if I did an identical study I would come out with same numbers. But I would still be cautious about drawing the conclusion that marijuana is responsible for those numbers," said Harrison Pope, a professor of psychiatry at Harvard Medical School.

That's because it's hard to figure out a chain of causality when doing marijuana studies. For example, long-term marijuana users are more likely to have spent high school stoned, and thus might be less likely to have boosted their vocabularies reading literature. So asking them to recall words they are not familiar with in the first place wouldn't necessarily mean the marijuana has damaged their brain.

"It's always and forever difficult to tease apart the confounding variables," said Pope, who has conducted research on marijuana use for more than 10 years.

In most aspects of science, the only way to answer a question once and for all is to do a randomized, controlled trial of 100 people or more, Pope said. But since giving people marijuana in a clinical setting poses a rather formidable dilemma, Pope said that he and other psychiatrists must fall back on messy methodology.

But Messinis is up for the task. Next he plans to explore two questions still up in the air: whether the deficits in memory and cognition are reversible, and also whether a heavier dose of marijuana -- not just duration -- has a different effect on the mind.

E-mail: consumerhealth@upi.com