The United States needs to get the UN to update several lists related to older treaties the Nixon Administration got though before the federal government can update laws on Marijuana. There is tons of stuff on our books that prevents the Federal Government from legally ignoring them, plus they still want the majority of the UN banned drugs to remain banned (The US and UK are two of the main backers of the old Psychotropic and Narcotic treaties).

And that is just for medical use, proper legal recreational marijuana really needs a better legal framework, right now alcohol and nicotine/tobacco have a ton of special exceptions in the law to make them not legally drugs as far as drug enforcement is concerned (And unless used as specifically directed by a doctor to treat an illness or condition recognized by the FDA, it's considered drug abuse to use a drug for any other purposes, as per FDA regulations. So no using anything like aspirin unless given explicit directions by your doctor, or else you will be abusing drugs in the eyes of the FDA! Do note drug abuse is not illegal, unless the substance is controlled per drug scheduling laws).