Based upon its current classification by the feds as a Class 1 narcotic, the feds have not allowed any widespread medical testing to take place. I can you first hand that when I use it for back pain and muscle spasms I get near instant relief and not some placebo effect.
There are scientists outside of the U.S. who are not constrained by our drug laws. If there were solid evidence that marijuana was useful as a medicine then we would know about it. Of course, it does not make sense to classify it as a narcotic and it’s debatable whether or not it should be illegal to be used for its intoxicating quality.