Smoking Marijuana Impairs Cognitive Function In MS Patients, Study Shows

"This is the first study to show that smoking marijuana can have a harmful effect on the cognitive skills of people with MS," said study author Anthony Feinstein, MPhil, PhD, of the University of Toronto. "This is important information because a significant minority of people with MS smoke marijuana as a treatment for the disease, even though there are no scientific studies demonstrating that it is an effective treatment for emotional difficulties."

Feinstein noted that MS itself can cause cognitive problems. "In addition, cognitive problems can greatly affect the quality of life for both patients and their caregivers," he said.

For the study, researchers interviewed 140 Canadian people with MS. Of those, 10 people had smoked marijuana within the last month and were defined as current marijuana users. The marijuana users were then each matched by age, sex, the length of time they had MS, and other factors to four people with MS who did not smoke marijuana.

The researchers then evaluated the participants for emotional problems such as depression, anxiety and other psychiatric disorders. They also tested the participants' thinking skills, speed at processing information, and memory.

The study found marijuana smokers performed 50 percent slower on tests of information processing speed compared to MS patients who did not smoke marijuana. There was also a significant association between smoking marijuana and emotional problems such as depression and anxiety.

People with MS have higher rates of depression and suicide compared to the general population. "Since marijuana can induce psychosis and anxiety in healthy people, we felt it was especially important to look at its effects on people with MS," Feinstein said.

This research was published February 13, 2008, in the online edition of Neurology®, the medical journal of the American Academy of Neurology. The study was supported by a grant from the Canadian Institutes for Health Research.