Obama has been fairly open about his history with marijuana use when he was younger, and since becoming the President he has even made remarks about it not being as dangerous as alcohol, but he has never taken a strong stand agains prohibition. President Obama wants to be clear that he does not support substance abuse, however he does think that marijuana should be treated similar to alcohol and tobacco. He says that legalization is not done by an order of the president, but instead should be handled by organizations in charge of enforcing prohibition or through legislation.
Typically how these classifications are changed are not done by presidential edict but are done either legislatively or through the DEA. As you might imagine, the DEA, whose job it is historically to enforce drug laws, is not always going to be on the cutting edge about these issues."
He added that it's "untenable over the long term for the Justice Department or the DEA to be enforcing a patchwork of laws, where something that's legal in one state could get you a 20-year prison sentence in another."