A study involving 50 people with MS (aged 18-65), of whom half used marijuana for pain relief, has found that marijuana users performed significantly worse on tests of attention, speed of thinking, executive function and visual perception of spatial relationships between objects. Those who used marijuana were also twice as likely as non-users to be classified as globally cognitively impaired.
The two groups were matched for age, gender, level of education, IQ before diagnosis, level of disability and duration of time with MS. On average, the duration of marijuana use was 26 years, and 72% reported smoking marijuana on a daily basis while 24% reported weekly use and one person reported bi-weekly use. There were no differences between the groups on measures of depression and anxiety.
And on a less-expected note, a study involving 40 people with multiple sclerosis and 40 people without MS has found those with MS scored 70% better on cognitive tests during cooler days compared to warmer days of the year. There was no link between test scores and temperature for those without MS.
 . Effects of cannabis on cognitive function in patients with multiple sclerosis. Neurology [Internet]. 2011 ;76(13):1153 - 1160. Available from: http://www.neurology.org/content/76/13/1153.abstract
The findings of the temperature study were presented at the American Academy of Neurology's 63rd Annual Meeting in Honolulu April 9 to April 16, 2011.