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Smoking Marijuana is Associated with Impaired Cognitive Function in Small Study of People with MS

February 15, 2008


Researchers have found that, in a small sample of people with MS who admitted smoking marijuana (Cannabis) regularly, measures of cognitive function indicate some impairment. This study does not address the question of whether smoking marijuana causes long-term cognitive impairment after use of the drug is discontinued. Omar Ghaffar, MD, FRCP(C) and Anthony Feinstein MPhil, PhD, FRCP(C) (Sunnybrook Health Services Centre, Toronto Canada) report their findings in the February 13 early online edition of the journal Neurology. This study adds to the growing body of literature on marijuana in MS. The National MS Society will soon publish an expert opinion paper on this subject that was written by a task force appointed to study the status of this ongoing area of MS research.

Details: Some people with MS report that smoking marijuana relieves several of their MS symptoms, such as spasticity and pain. However, scientific studies completed thus far have not generally provided convincing evidence that marijuana or its derivatives provide substantiated benefits for symptoms of MS. The fact that marijuana is an illegal drug in many states and by federal statute further complicates the issue.

Cognitive impairment is a troublesome symptom experienced by a significant portion of people with MS. This study asks important questions about whether use of marijuana by people with MS increases the risk of cognitive impairment.

In this study, Drs. Ghaffar and Feinstein evaluated 140 consecutive patients with MS using a standardized psychiatric diagnostic interview and a battery of neuropsychological tests. Among this group, 95 people said that they had never used illicit drugs; 32 admitted that they had used cannabis at least once; and 10 were classified as being current users who smoked marijuana frequently (daily, weekly, or monthly) and had used it last from one to 30 days before the interviews. Psychiatric and cognitive test results for each of these 10 patients were compared with four matched controls from the group who had never used marijuana, 40 in all.

In that comparison, the proportion of patients meeting the criteria for any psychiatric diagnosis was higher in the 10 cannabis users than in the matched non-users. Also, users had a slower performance time on a test of information processing speed, working memory, and sustained attention.

Although this study suggests that smoking marijuana is associated with impaired cognition in people with MS, further study is necessary to determine if using cannabis causes the impairment. The authors note some limitations of the study, such as the small sample size and the fact that marijuana use was assessed via patient report and not by urine toxicology testing. Also, marijuana is known to cause short-term memory and other cognitive impairments in users in the general population, but studies of how permanent such impairment is once a person stops using the drug have had mixed results, some suggesting no lasting impairment and some suggesting some residual cognitive impairment. This study was not designed to evaluate that question in users who have MS.

This study adds to the growing body of literature on cannabis use in MS. The National MS Society recently appointed a task force to discuss the status of research on this topic, and this group will soon publish an expert opinion paper.

-- Research and Clinical Programs Department


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