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.