Dylan Matthews explains what the president could do if he were serious about drug reform:

[T]here’s a way for the Obama administration to decriminalize medical marijuana without even involving Congress. The Controlled Substances Act, the 1970 law that governs federal drug policy, is based on a system known as "scheduling," in which drugs are sorted into categories based on their potential for abuse and usefulness in medicine. Marijuana, along with the likes of heroin and DMT, is a schedule I drug, meaning it is judged to have a high potential for abuse and little medical value. By contrast, cocaine, oxycontin and PCP are all schedule II drugs, and can be prescribed.

Rulings on scheduling, however, are not permanent. Upon petition from private citizens, the DEA can initiate a process that results in a drug being rescheduled. In effect, that means that the attorney general can direct the DEA to act on a petition for marijuana rescheduling. In effect, Eric Holder could direct the agency to remove marijuana from the list of scheduled drugs, decriminalizing it for medical use federally. That doesn’t help recreational users, but it would let medicinal users and suppliers breathe a lot easier. While states could still ban it for medicinal use, those that opt not to would no longer run afoul of federal law.