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Brain Damage Marijuana

Heavy Marijuana Use Linked to Brain Damage
News Article
March 6, 2002

Researchers report that chronic users of marijuana suffer memory loss and attention problems that can affect their work, their life, and their ability to learn. However, a medical expert not involved in the study questioned the findings and whether the alleged adverse impact of marijuana is really there, which is indicative of the controversy surrounding the drug.

The findings are published in the current Journal of the American Medical Association. The study was based on patients seeking help for marijuana dependence at clinics in Seattle, Farmington, Connecticut, and Miami, between 1997 and 2000.

Fifty-one people were examined who had been using marijuana regularly for an average of 24 years. Also, 51 short-term users and 33 nonusers were included as controls for comparison purposes in the research. "Long-term users ... performed significantly less well on tests of memory and attention than nonuser controls and shorter-term users with a mean of 10 years' use," the study said.

On a verbal learning test "long-term users recalled significantly fewer words than either shorter-term users or controls; there was no difference between shorter-term users and controls. Long-term users showed impaired learning, retention and retrieval compared with controls," the study said.

The study's authors, including Nadia Solowij at the University of New South Wales, Sydney, Australia, and colleagues with the Marijuana Treatment Project Research Group said the findings confirmed and expounded upon previous findings of cognitive impairments among chronic users. "For habitual users, the kinds of impairments observed in this study have the potential to impact academic achievements, occupational proficiency, interpersonal relationships and daily functioning," said the study.

However, in an editorial in the same issue, Harrison Pope of Harvard Medical School said "a recent meta-analysis of neuropsychological studies of long-term marijuana users found no significant evidence for deficits in seven of eight ... ability areas and only a small effect size for the remaining area of learning."

He said in a separate statement that the study does not explore whether the heavy users may have been taking other drugs that could have accounted for the deficits uncovered, or whether they might have been suffering from anxiety or depression that could cause the problems noted.

"Another recent study from our laboratory ... found virtually no significant differences between 108 heavy cannabis users and 72 controls -- screened to exclude those with current psychiatric disorders, medication use, or any history of significant use of other drugs or alcohol -- on a battery of ten neuropsychological tests after 28 days of supervised abstinence from the drug," he said.

Pope, the director of the Biological Psychiatry Laboratory at McLean Hospital in Belmont, Massachusetts, added: "The safest thing to say at this point is that the jury is still out on the question of whether long-term marijuana use causes lasting impairment in brain function."


California NORML Reports, April 1992

Two new scientific studies have failed to find evidence of brain damage in monkeys exposed to marijuana, undercutting claims that marijuana causes brain damage in humans.

The studies were conducted by two independent research groups. The first, conducted by Dr. William Slikker, Jr. and others at the National Center for Toxicological Research in Arkansas examined some 64 rhesus monkeys, half of which were exposed to daily or weekly doses of marijuana smoke for a year. The other, by Gordon T. Pryor and Charles Rebert at SRI International in Menlo Park, California, which is still unpublished, looked at over 30 rhesus monkeys that had inhaled marijuana one to three times a day over periods of 6 to 12 months. Neither study found evidence of structural or neurochemical changes in the brains of the monkeys when examined a few months after cessation of smoking.

The new results cast doubt on earlier studies purporting to show brain damage in animals. The most famous of these was a study by Dr. Robert Heath, who claimed to find brain damage in three monkeys heavily exposed to cannabis. Heath's results failed to win general acceptance in the scientific community because of the small number of subjects, questionable controls, and heavy doses.

Subsequent rat experiments by Dr. Slikker and others reported persistent structural changes in the brain cells of rats chronically exposed to THC. The studies did not show that pot kills brain cells, as alleged by some pot critics, but they did show degeneration of the nerve connections between brain cells in the hippocampus, where THC is known to be active. Although scientists have regarded the animal evidence as inconclusive, some critics have cited it as proof that pot causes brain damage in humans. Thus Andrew Mecca, the director of California Department of Alcohol and Drug Abuse, recently stated on the Ron Reagan, Jr. talk show (Sep. 2, 1991) that marijuana "leaves a black protein substance in the synaptic cleft" of brain cells, a claim apparently based on Heath's monkeys. When asked by a NORML member for his evidence, Mecca sent a list of three references, none of which turned out to have anything to do with brain damage.

Although the new monkey studies found no physical brain damage, they did observe behavioral changes from marijuana. Slikker's group found that monkeys exposed once a day to the human equivalent of four or five joints showed persistent effects throughout the day. Slikker says that the effects faded gradually after they were taken off marijuana, and were not detectable seven months later, when they were sacrificed. Autopsies did reveal lingering chemical changes in the immune cells in the lungs of monkeys that had inhaled THC. However, Slikker's group concluded that experimental exposure to marijuana smoke "does not compromise the general health of the rhesus monkey."

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