Prohibited substance Even though marijuana is widely known as a prohibited substance, there are some places in the world that have already legalized it. In the United States, there are more than 20 states that have passed their marijuana laws and bills to allow marijuana for medical purposes. In order to avail of the medical marijuana treatment in Michigan, you should be diagnosed with any of the approved conditions: nail patella, epilepsy, multiple sclerosis, Hepatitis C, HIV, AIDS, ...