Legalize it and tax it. It's no worse for you than alcohol or cigarettes. There is a lot of hypocrisy surrounding this issue.
In terms of medical evidence, if people who are suffering say it helps, then that is enough for me. It's not fair for others who don't understand the pain of these illnesses to make the moral choice for people suffering.
People using it for medical reasons are hurting no one. They aren't trafficking in the drugs. Money could be much better spent by leaving these people alone.