Medical Marijuana for Pain: What the Evidence Shows

Overall, these studies seem to indicate that cannabinoids have a significant role to play in the management of chronic pain. However, there are important issues that limit the validity of this conclusion. First and most important is how the improvement in pain was evaluated. In many of the studies, only instruments to measure the level of pain, most notably the Visual Analogue Scale, were used. This is fine when one is measuring acute pain. But when it comes to chronic pain—which is what the studies were looking at—the most important measures of the impact of any treatment are improvement in functioning and other objective measures, such as reduction in use of analgesic medications.
he review found minimal evidence that cannabinoids are beneficial for the nausea and vomiting associated with chemotherapy or for stimulation of appetite in patients with HIV/AIDS. It also found no evidence of effectiveness for the treatment of glaucoma or mental disorders; instead, it found that cannabinoids were more likely to worsen the latter. There was some evidence that cannabinoids might be beneficial for the spasticity associated with multiple sclerosis, but even here the results did not reach statistical significance. 
Current research has fallen far short of criteria required by the FDA to approve any drug, much less for conditions not considered acutely life-threatening and for which there are already many other effective and safe treatments. The variation from state to state as to which medical conditions are approved for cannabinoid use is not because lawmakers have ignored scientific evidence. Rather, it reflects the absence of such evidence and an approval process based on politics, not science.