WEDNESDAY, Feb. 13 (HealthDay News) -- Multiple sclerosis patients who smoke marijuana in search of symptom relief are more likely to suffer cognitive shortfalls and mood disorders, new Canadian research suggests.

A slowing down in the ability to process and remember information is one significant side effect, as is a rise in the rate of depression and anxiety.

"This is a small study, so our findings are preliminary, but the bottom line is that multiple sclerosis patients who smoke cannabis appear to be at an increased risk for cognitive issues, particularly with respect to the speed of their thinking," said study author Dr. Anthony Feinstein, a professor of psychiatry with the Sunnybrook Health Sciences Centre's department of psychiatry at the University of Toronto.

Feinstein's observations are published in the Feb. 13 online edition of Neurology and are focused exclusively on the impact of smoking marijuana illegally obtained by patients themselves. Medically prescribed marijuana was not studied.

The authors noted that a "significant minority" of multiple sclerosis patients smoke marijuana to combat the tingling, numbness, blindness and paralysis that can accompany the progressive and often disabling nervous system disease.

However, Feinstein's team stressed that scientists have yet to definitively prove that the psychoactive substance -- long linked to psychosis, anxiety and delirium among healthy users -- provides a measurable benefit to the more than 400,000 Americans and 2.5 million people worldwide who suffer from the disease.

The researchers therefore assessed the experience of 140 Toronto-based MS outpatients, 10 of whom had smoked the drug at least once in the previous month and were considered regular marijuana users.

All the patients -- three-quarters of them women -- underwent cognitive and mental health exams by a neurologist and a neuro-psychiatrist. Interviews were also conducted to assess disease severity and course, medications being used, and current disability.

Feinstein and his team observed that while pot smokers were younger, there were no differences between marijuana users and nonusers in terms of gender, education, or MS disease course or duration.

However, MS patients who used marijuana were found to perform 50 percent slower on tests tracking information-processing speed and were more likely than nonusers to have a mental disability of some kind.

Marijuana use was also associated with a greater risk for being depressed or experiencing anxiety. However, the authors were not able to determine whether the drug had triggered such conditions, or if patients had sought out marijuana to help deal with a preexisting emotional issue.

They nonetheless cautioned that smoking marijuana might further raise the risk for experiencing the kind of neuro-psychological impairment that typically occurs among 40 percent to 65 percent of all MS patients.

Feinstein said that he next hopes to gather a much larger pool of patients, while exploring possible differences in the health impact of street-purchased marijuana versus prescribed cannabis.

Meanwhile, Dr. Marshall Keilson, director of neurology at Maimonides Medical Center in Brooklyn, N.Y., said he thinks it best to proceed on a case-by-case basis.

"There are some MS patients who are emotionally disabled from their disease, and if we can use cannabis to help them feel better about the world or life, we should," he said. "We need to always err on the side of doing what's best for our patients. And I don't necessarily believe there is a permanent damage to the brain, based on occasional marijuana use. If they're smoking 10 times a day, yes, there will be damage done. But this goes for excessive alcohol use, too. So, I think we're going to end up somewhere in the middle with this."

More information

For more on multiple sclerosis, visit the National Multiple Sclerosis Society.

SOURCES: Anthony Feinstein, M.D., Ph.D., professor, psychiatry, department of psychiatry, Sunnybrook Health Sciences Centre, University of Toronto, Ontario, Canada; Marshall Keilson, M.D., director, neurology, Maimonides Medical Center, Brooklyn, N.Y.; Feb. 13, 2008, Neurology, online