I have no problem with medical Marijuana. What I have a problem with is recreational usage of it. First, it impairs your judgement, thereby endangering yourself and others. If there was a way to detect Marijuana in the system like a breathalyzer detecting alcohol, I would not have much of a problem with it. The second reason is that it deprives one of a fully informed free will. When you are under the influence of those kind of drugs, you can't discern moral situations with as much clarity.