Heavy Marijuana Use Doesn't Damage Brain

Analysis of Studies Finds Little Effect From Long-Term Use

July 1, 2003 -- Long-term and even daily marijuana use doesn't appear to cause permanent brain damage, adding to evidence that it can be a safe and effective treatment for a wide range of diseases, say researchers.

The researchers found only a "very small" impairment in memory and learning among long-term marijuana users. Otherwise, scores on thinking tests were similar to those who don't smoke marijuana, according to a new analysis of 15 previous studies.

In those studies, some 700 regular marijuana users were compared with 484 non-users on various aspects of brain function -- including reaction time, language and motor skills, reasoning ability, memory, and the ability to learn new information.

"We were somewhat surprised by our finding, especially since there's been a controversy for some years on whether long-term cannabis use causes brain damage," says lead researcher and psychiatrist Igor Grant, MD.

"I suppose we expected to see some differences in people who were heavy users, but in fact the differences were very minimal."

The marijuana users in those 15 studies -- which lasted between three months to more than 13 years -- had smoked marijuana several times a week or month or daily. Still, researchers say impairments were less than what is typically found from using alcohol or other drugs.

"All study participants were adults," says Grant, professor of psychiatry and director of the Center for Medicinal Cannabis Research Center at the University of California, San Diego School of Medicine.

"However, there might be a different set of circumstances to a 12-year-old whose nervous system is still developing."

Grant's analysis, published in the July issue of the Journal of the International Neuropsychological Society, comes as many states consider laws allowing marijuana to be used to treat certain medical conditions. Earlier this year, Maryland became the 10th state to allow marijuana use to relieve pain and other symptoms of AIDS, multiple sclerosis, cancer, glaucoma, and other conditions -- joining Alaska, Arizona, California, Colorado, Hawaii, Maine, Nevada, Oregon, and Washington.

Medicinal marijuana is available by prescription in the Netherlands and a new marijuana drug is expected to be released in Great Britain later this year. In the U.S. and elsewhere, Marinol, a drug that is a synthetic form of marijuana and contains its active ingredient, THC, is available by prescription to treat loss of appetite associated with weight loss in AIDS patients.

Grant says he did the analysis to help determine long-term toxicity from long-term and frequent marijuana use. His center is currently conducting 11 studies to determine its safety and efficacy in treating several diseases.

"This finding enables us to see a marginal level of safety, if those studies prove that cannabis can be effective," Grant tells WebMD. "If we barely find this effect in long-term heavy users, then we are unlikely to see deleterious side effects in individuals who receive cannabis for a short time in a medical setting, which would be safer than what is practiced by street users."

Grant's findings come as no surprise to Tod Mikuriya, MD, former director of non-classified marijuana research for the National Institute of Mental Health Center for Narcotics and Drug Abuse Studies and author of The Marijuana Medical Handbook: A Guide to Therapeutic Use. He is currently president of the California Cannabis Medical Group, which has treated some 20,000 patients with medicinal marijuana and Marinol.

"I just re-published a paper of the first survey for marijuana toxicity done in 1863 by the British government in India that was the most exhaustive medical study of its time in regards to possible difficulties and toxicity of cannabis. And it reached the same conclusion as Grant," Mikuriya tells WebMD.

"This is merely confirming what was known over 100 years ago, as well as what was learned by various government findings doing similar research -- marijuana is not toxic, but it is a highly effective medicine."

In fact, marijuana was available as a medicinal treatment in the U.S. until the 1930s.

Lester Grinspoon, MD, a retired Harvard Medical School psychiatrist who studied medicinal marijuana use since the 1960s and wrote two books on the topic, says that while Grant's finding provides more evidence on its safety, "it's nothing that those of us who have been studying this haven't known for a very long time.

"Marijuana is a remarkably safe and non-toxic drug that can effectively treat about 30 different conditions," he tells WebMD. "I predict it will become the aspirin of the 21st century, as more people recognize this."