Although medical marijuana has been legalized in many U.S. states, people with multiple sclerosis should think twice before they start using the drug routinely, researchers say.
In a small study published Monday, they found people with the nerve-damaging autoimmune disease did worse on a number of psychological tests if they were heavy marijuana smokers.
"In multiple sclerosis, you already have a group of patients who are cognitively impaired," said Dr. Anthony Feinstein, who led the new work. "When you add marijuana to the mix, you might worsen those problems."
Still, the study doesn't prove that marijuana fuels mental decline, Feinstein said. And even if it does, some people might choose to live with that trade-off.
Many people with chronic diseases say the illegal drug helps relieve their symptoms, and a few studies have hinted that cannabis extracts might alleviate pain, spasticity and other problems in multiple sclerosis (MS).
The disease gnaws away at the fatty sheath around nerve cells, which can cause severe symptoms like vision loss, numbness, tremors, muscle stiffness and mental changes.
According to the National MS Society, some 400,000 Americans have MS. Of those, about one in six smoke marijuana, Feinstein said, yet almost no research has looked at how it affects their minds.
"There are just no data on the topic," Feinstein, a psychiatrist at the University of Toronto, told Reuters Health.
So he and his colleagues decided to compare the mental skills of two groups of 25 people with MS. One group was made up of regular marijuana users -- the majority had smoked it daily for many years -- and the other included only non-users.
The two groups were matched on age, stage and course of disease, education and other factors.
Nearly two-thirds of the marijuana smokers were classified as cognitively impaired based on a several psychological tests, such as information processing speed and verbal memory (all taken at least 12 hours after the patients had last used marijuana).
By comparison, only a third of the non-users had similar mental impairment.
The effect was independent of other factors that might affect mental functioning - such as whether the participants drank alcohol or were depressed or anxious.
Still, short of running an actual experiment that administers the drug randomly to volunteers, it's impossible to prove that marijuana use was at the root of the lower mental performance seen in the regular pot smokers.
"It's not really surprising," said Dr. Shaheen Lakhan of the Cleveland Clinic in Ohio, who also runs the non-profit Global Neuroscience Initiative Foundation in Panorama City, California.
"It's a long established observation that there are negative cognitive effects on cannabis users," Lakhan said. "But I must say that most MS patients that I am familiar with find that their pain and spasticity are very disturbing, and they might choose this as a priority."
He added that there are several medications approved to treat MS symptoms, including muscle relaxants, though all have side effects.
Feinstein said people with MS should be careful about smoking marijuana, especially because the therapeutic benefits appear to be weak.
Still, he said, people in different situations may have different priorities.
"I don't want to be dogmatic about it," Feinstein told Reuters Health. "It's really going to vary with the individual patient."