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Multiple Sclerosis Health Center

Medical Marijuana May Impair Thinking of MS Patients

Study Shows Cognitive Impairment May Be an Issue for Long-Term Users of Medical Marijuana
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Marijuana Use and Mental Performance continued...

The average duration of marijuana use among the users studied was 26 years. Seventy-two percent of users reported using marijuana daily; 24% reported weekly use.

All study participants underwent a series of tests used to measure various aspects of mental functioning, including working memory, speed of information processing, and visual perception of spatial relationships. Study participants who used marijuana were not tested if last use was less than 12 hours before testing.

Feinstein says the marijuana users performed significantly worse than the non-users on tests measuring attention, speed of thinking, visual perception, and cognition related to planning and organizing.

Scores on one test measuring speed of processing information were about a third lower among marijuana users compared to non-users.

Thirty-two percent of non-users and 64% of users met the definition of globally cognitively impaired, meaning that they had measurable impairments in two or more aspects of intellectual functioning.

Medical Marijuana Debate

The researchers say larger studies are needed to confirm their findings.

Neurologist Lily Jung Hensen, MD, of Seattle’s Swedish Neurosciences Institute, tells WebMD that the findings make a strong argument that the cognitive risks associated with marijuana use outweigh potential benefits for MS patients.

Morgan Fox of the Marijuana Policy Project notes that the researchers did not consider other medications that the study participants might have been taking and he said the study had other limitations that could have affected the findings.

”No study has been able to show long-term, permanent cognitive damage in adult marijuana users, only temporarily decreased abilities,” he tells WebMD.

Fox noted that all medications have side effects, but decisions about whether or not to use them should be left to patients and their doctors.

Williams agrees, adding that a review of 15 medical marijuana studies conducted over the last 20 years showed no evidence of long-term cognitive decline. The review, conducted by researchers from the University of California’s Center for Medicinal Cannabis Research, was reported in February 2010.

“Marijuana is not going to work for everybody, but no pain medication on the planet does,” he says. “But if my doctor can write me prescriptions for morphine and for OxyContin, he should certainly be able to write me a prescription for this drug.”

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