As President Obama could order the DEA to reschedule
marijuana as a Schedule 2 drug. Msarijuanas is classified as
a Scheedule 1
drug is not a federal law.
It is a internal ruling by the DEA
and hence can be easily recalassified if Obama so orders it.
The US Attorney General can initiate rescheduling on his or her own, at the request of the Health and Human Services Secretary, on petition of interested party, or if obligated by international treaties.
The US Secretary of Health and Human Services can issue a binding recommendation to the Attorney General to reschedule marijuana.
US Congress can amend the Controlled Substances Act it passed in 1970 to reschedule marijuana.
The DEA (Drug Enforcement Administration) and FDA (Food and Drug Administration) together can reschedule marijuana if clinical trials show that it is "safe" and "effective."
A law passed by Congress: this has been atteempted several times but always failed.