Cannabis Use May Harm Cognitive Function in MS Patients

Megan Brooks

March 28, 2011

March 28, 2011 — Prolonged heavy use of street cannabis may adversely affect cognitive function in patients with multiple sclerosis (MS), results of a cross-sectional study hint.

After controlling for potential confounding factors, long-term cannabis users were roughly twice as likely as nonusers to meet criteria for global cognitive impairment, defined as impairment on 2 or more aspects of intellectual functioning.

"Whatever subjective benefits patients may derive from using street cannabis (i.e., pain and spasticity relief) should be weighed against the associated cognitive side effects," the study team concludes.

Their report appears in the March 29 issue of Neurology.

In an email to Medscape Medical News, study investigator Anthony Feinstein, MD, PhD, from Sunnybrook Health Sciences Centre and University of Toronto, Canada, said, "Smoking cannabis may exacerbate cognitive problems that are already present in 40% to 60% of MS patients."

Dr. Anthony Feinstein

The cognitive deficits "are potentially quite widespread, involving information processing speed, visuospatial abilities, attention, and executive function," Dr. Feinstein added.

Interpret With Caution

But in a telephone interview, Helen Tremlett, PhD, of University of British Columbia in Vancouver, Canada, who was not involved in the study, cautioned that it was small and cross-sectional, "so causation can not be assumed." The fact that these were also heavy, long-term cannabis users is also important to note — both points echoed by the study team in their paper as well.

Dr. Helen Tremlett

The study involved 50 patients with MS, who were, on average, 43 years old and had had MS for about 12 years. Twenty-five used cannabis (smoked or ingested) and the other 25 did not.

On average, cannabis users started using the drug at age 17 and used it for an average of over 26 years. Eighteen (72%) used cannabis daily, 6 (24%) used it weekly, and 1 reported biweekly use.

The cannabis users and nonusers were well matched in terms of sex, level of education, IQ before diagnosis, and level of disability. Results of a broad-spectrum urine drug screen indicated that no participant used any illicit drugs other than cannabis.

Compared with nonusers, cannabis users scored significantly lower on a number of standard cognitive tests measuring visuospatial perception, executive functioning, and information processing speed.

In addition, among all 50 patients, 24 (48%) were classified as cognitively impaired, and cannabis users were significantly more likely to be classified as globally impaired compared with nonusers (P = .024). Global cognitive impairment was not significantly correlated with urine cannabinoid levels or age at which cannabis use started (P = .118 for both) or duration of use (P = .451).

The investigators note that the cognitive tests were administered more than 12 hours after the last cannabis use.

"The literature from the general population suggests, with few exceptions, that there are residual, adverse cognitive difficulties extending beyond this period," the investigators note. "Our finding in the cannabis users replicates this picture and points toward the detrimental effects of cannabis persisting beyond intoxication."

Not Typical MS Cannabis Users

Dr. Tremlett noted that some of the cannabis users had started smoking cannabis well before onset of their MS. "They didn't start smoking cannabis because they developed MS and thought it would help," she pointed out.

In addition, some of the cannabis users were smoking up to several times a day for over 2 decades, starting in their teens. "These are heavy cannabis users, which are likely different from your typical MS patients," Dr. Tremlett said. "I would certainly not see them as representative of cannabis users in the general MS community."

Echoing this point, the study investigators emphasize in their report that the results don't necessarily extend to occasional cannabis use or frequent use for a brief period of time. The fact that they were smoking street cannabis, the quality of which is unknown, is another potential limitation of the study.

Strengths of the study, Dr. Tremlett said, include publication in a peer-reviewed journal by "well-qualified researchers." The fact that they used validated screening tools for depression and cognition and a urine test to rule out people who had used other illicit drugs are also strengths.

Despite these caveats, Dr. Feinstein said patients who smoke cannabis should be aware of the potential for cognitive problems, and "so too should neurologists and healthcare providers involved with MS patients' care."

Dr. Tremlett agreed. "There should be an open dialogue between MS patients and their healthcare providers about what they are using, that's important," she said.

The study was supported by the Multiple Sclerosis Society of Canada. Dr. Feinstein discloses financial relationships with several pharmaceutical companies, including Merck Serono, Avanir Pharmaceuticals, Teva Pharmaceuticals, Bayer Schering Pharma, and Biogen Idec. A complete list of disclosures for all of the investigators can be found with the original article.

Dr. Tremlett is funded by the Multiple Sclerosis Society of Canada, is a Michael Smith Foundation for Health Research Scholar, and the Canada Research Chair for Neuroepidemiology and Multiple Sclerosis. She has also received research support from the US National Multiple Sclerosis Society, Canadian Institutes of Health Research, and the United Kingdom MS Trust and speaker honoraria or travel expenses to speak at conferences from the Consortium of MS Centres, US National MS Society, Swiss Multiple Sclerosis Society, the University of British Columbia Multiple Sclerosis Research Program, Bayer Pharmaceutical (2010, invited speaker, honoraria declined), and Teva Pharmaceuticals (2011, invited speaker). Unless otherwise stated, all speaker honoraria are donated to an MS charity or to an unrestricted grant for use by her research group.

Neurology. 2011;76:1153-1160. Abstract

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