What the executive branch can do without assistance from the legislative branch is change marijuana from a Schedule I drug to a Schedule III or IV drug. Schedule I drugs are defined as drugs which have no currently accepted medical use in the United States, a lack of accepted safety for use under medical supervision, and a high potential for abuse.
There seems to be some reasonably strong evidence that marijuana does have currently acceptable medical use.
Once a drug gets off of the Schedule I list, it can at least be prescribed under some circumstances.
Thanks - I believe that medical marijuana has come under such close scrutiny because the folks who dispense it have found ways to make it a true OTC product w/o the controls put on many other prescription drugs.