Based on the numbers of young children dying due to prescription opiate abuse, I think that it would be smarter to first prescribe marijuana for severe pain.
The pharmacology of marijuana and opiates is truly apples to oranges. One can kill you, the other cannot. One causes constipation, side effects, depression, addiction, and physical withdrawal. Not to mention the neuroscience of opiates is unpredictable- reformed users have been known to alter their tolerance level based on place preference.
As far as states rights, no. THe repubs are not going to stand for this. The big pharma drugs have enough $$$ in their pockets. Making marijuana legal would nullify a shitload of opiates, antianxiety, and antidepressant meds out there. Big Pharma has a vested interest in keeping down a plant that is effective, close to free, and grows well in almost any condition.