Over the last two years, the United States has witnessed enormous change concerning the acceptance of marijuana. The number of persons with MS (PwMS) using cannabis to treat their symptoms is expected to rise, and it is important to provide clinicians and PwMS with evidence based information on the safety and effectiveness of marijuana. In addition, scientists need to identify the effects of medical marijuana as the compound action of different cannabinoids, specifically tetrahydrocannabinol (THC) and cannabidiol (CBD). The ratio of CBD to THC may have an important impact on the effects of the marijuana, which in turn may have important implications for the medical benefits. The overarching goal of this study is to advance a better understanding of the potential positive and negative effects associated with different strains of marijuana, using an interventional design. The investigators will examine three strains that differ markedly on THC potency (A. THC <1%, CBD > 10%; B. THC >10%, CBD < 1%; C. THC 510%, CBD 510%) to determine whether the level of THC is associated with differences in motor and cognitive functions in PwMS. One hundred PwMS will be recruited for this project and followed for 3 months following designation to one of three strains of marijuana with different ratios of CBD to THC or a placebo marijuana (THC 0%, CBD 0%). It is hypothesized that the strains with a high CBD to THC ratios will demonstrate the most benefits on measures of motor and cognitive function.