Pauline Anderson

May 28, 2015

INDIANAPOLIS, Indiana — About 40% of patients with multiple sclerosis have used cannabis, some to relieve pain, control spasms, or improve sleep, but research shows that it has negative consequences in terms of cognition.

Neurologists should inform their patients with MS who are smoking cannabis that a possible adverse effect of the habit is "enduring" impaired cognition, said neuropsychiatric expert Anthony Feinstein, MD, PhD, professor, psychiatry, University of Toronto, Ontario, Canada.

Dr Feinstein, who has researched the effects of smoking marijuana on the brain, addressed a media briefing and delivered the opening lecture on the subject during the Consortium of Multiple Sclerosis Centers (CMSC) 2015 Annual Meeting here.

In many cases, having MS spurs people to try cannabis; research cited by Dr Feinstein shows that over half of patients with MS who have used pot first tried it after getting their diagnosis. And about 75% of patients with MS who have never used cannabis say they would do so if it were legal. Most patients who use the drug smoke it in cigarettes or pipes.

Unlike the demographics of MS in general, the patient with MS who smokes marijuana is typically male and a tobacco user. Many of these patients use pot frequently, sometimes two to three times a day and often five to six days a week, which, as Dr Feinstein pointed out, can be costly — up to $500 per month.

No Hard Evidence

The literature isn't clear on how smoking marijuana affects patients with MS. A 2014 American Academy of Neurology (AAN) position paper concluded that there's no hard evidence on the benefits or harms of cannabis in MS. "So basically, they're calling for more research," said Dr Feinstein.

The disease itself often affects cognition. From 40% to 70% of patients with MS have some impairment in information processing speed, working memory, visual-spatial memory, and/or executive function. Dr Feinstein's own research shows that smoking marijuana can make those deficits worse.

"We have shown that if they start smoking cannabis, the numbers go up; it aggravates preexisting cognitive deficits," he said.

One of his studies showed that persons who had smoked marijuana were about 50% slower on a single-digit test than nonsmokers. Another larger study that tested urine for cannabis metabolites also found "quite striking" cognitive differences between cannabis smokers and nonsmokers.

One study looked at working memory while participants were being scanned. The test required participants to press a button if a designated letter reappeared. The responses in the cannabis group were significantly slower.

Similar findings were found for information processing speed, which Dr Feinstein called the "quintessential cognitive problem" in patients with MS.

"The brain is activated fundamentally differently" in patients with MS who smoked cannabis compared with those who didn't, he concluded.

The "interesting thing" is that other research suggested that there are no significant structural changes in the brain from cannabis use, said Dr Feinstein.

He stressed that these results need to be replicated.

Adolescents Vulnerable

The life stage at which people in general start smoking cannabis appears to be important for cognition. Dr Feinstein cited a 2012 New Zealand study that followed over 1000 participants for 25 years, checking cannabis use along the way. The researchers found that people who started smoking pot during adolescence and used it fairly regularly (defined as more than four times a week) had significant cognitive deficits by the time they were in their 30s.

"Some of those deficits don't return," said Dr Feinstein. "It seems pretty clear that a very vulnerable group are adolescents."

But if they didn't pick up the habit during adolescence, persons without MS who quit smoking can return to the same cognitive level they had before. The "big unknown question," however, is whether cognition goes back to baseline for patients with MS if they stop smoking cannabis, said Dr Feinstein. "We don't know that yet."

Dr Feinstein noted that it's difficult to study the effect of smoking marijuana on cognition. There are multiple confounders, including the duration and frequency of use and the potency of the strain smoked.

Attitudes toward smoking pot have changed drastically in recent years. "I think now that a slight majority would say there's nothing wrong with smoking cannabis, said Dr Feinstein, who himself acknowledged having "changed his tune over the years" on the topic.

Asked his opinion of patients with MS using cannabis, Michael Racke, MD, professor, neurology, Ohio State University, Columbus, pointed out that it can cause significant lung damage, is still illegal in many places, and can affect day-to-day functioning.

"I can take my injection of interferon and go and drive my car, and it's no big deal, but if I smoke a joint and drive my car, I might be stopped for driving under the influence."

But he agreed that attitudes about smoking cannabis have changed significantly, with several states, Washington and Colorado, for example, having legalized it.

Dr Feinstein's research is supported by the MS Society of Canada.

Consortium of Multiple Sclerosis Centers (CMSC) 2015 Annual Meeting. Opening lecture. Presented May 27, 2015.


Comments on Medscape are moderated and should be professional in tone and on topic. You must declare any conflicts of interest related to your comments and responses. Please see our Commenting Guide for further information. We reserve the right to remove posts at our sole discretion.