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Marijuana Use Hurts Thinking Skills of MS Patients, Study Finds

March 28 (Bloomberg) -- Smoking marijuana to relieve symptoms of multiple sclerosis, such as pain or muscle spasms, may harm thinking skills of patients with the neurological disorder, a Canadian study found.

MS patients who smoked marijuana scored “significantly” lower than patients with the disorder who didn’t use the drug in tests of attention, thinking speed and gauging space between objects, according to research published today in the journal Neurology. The study compared 25 cannabis users with 25 non-users.

Multiple sclerosis, a debilitating neurological disorder, affects about 400,000 people in the U.S. and 2.5 million worldwide, according to the National Multiple Sclerosis Society. About 40 percent to 60 percent of people with MS experience problems with decision making, thinking and reasoning, the researchers wrote. Smoking marijuana may be worsening those effects, study author Anthony Feinstein said.

“There’s a real chance that the cannabis is going to worsen cognitive deficits that are already present,” said Feinstein, a professor of psychiatry at the University of Toronto and at Sunnybrook Health Sciences Center, in a telephone interview. “It’s kind of adding to the burden of the disease. You have to be very cautious when using this drug. You shouldn’t be using it at all.”

The study found that patients who used marijuana were twice as likely as non-users to be cognitively impaired, meaning they scored low on two or more test areas including information processing speed and problem solving. Marijuana users also scored about one-third lower on a test that measured the speed at which they processed information.

MS Marijuana Uses

MS patients use marijuana sometimes to alleviate pain and bladder dysfunction. It’s unclear how it affects reasoning and thinking, he said. Feinstein is planning a study using MRI, or magnetic resonance imaging, to track how marijuana affects the brain in MS patients who smoke it.

In the study, patients using cannabis were tested 12 hours or more after they last took the drug. Those in the marijuana group used the drug for an average of 26 years. Most of them used it daily for medical and recreational reasons, the paper said.

Data show that 36 percent to 43 percent of MS patients have smoked marijuana at some time, and about 14 to 18 percent are current cannabis users, the researchers said.

Multiple sclerosis symptoms can range from benign to disabling, according to the U.S. National Institutes of Health. In those with the disease, communication between the brain and other parts of the body is disrupted. MS is thought to be an autoimmune disease where the body attacks its own tissue, the agency said. There is no cure.

To contact the reporter on this story: Nicole Ostrow in New York at

To contact the editor responsible for this story: Reg Gale at

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