Caveat: I'm unfamiliar with the legality of "medical marijuana".
It's legal by state law. So you don't generally have to worry about the state po-po getting in your face about it.
However, Marijuana is still a "schedule 1" drug per the DEA and federal law, which means that the
feds, legally speaking, don't recognize any 'valid medical use'.
Note, even cocaine is only schedule 2 - highly abusable, but a few valid medical uses. I understand it's still useful in very specific situations for eyes (a drop or so of cocaine solution onto the eyeball).
That being said, the DEA and FBI aren't equipped to go after regular users - they depend on the states to do that. They're after the shippers and larger dealers.