Washington: A Canadian research has revealed that any possible pain relief that marijuana has for people with multiple sclerosis (MS) may be outweighed by the drug``s apparent negative effect on thinking skills. Some clinical trials have reported a mild benefit of marijuana on pain, bladder dysfunction and spasticity in MS, an auto-immune disease that affects the brain and spinal cord. The researchers studied two groups of 25 people each between the ages of 18 and 65 with MS. One group used marijuana and the other reported no marijuana use for many years. Urine tests were used to confirm use or non-use of the drug. The groups were matched so there would not be significant differences due to age, gender, level of education, IQ before diagnosis, level of disability and duration of time with MS.