The states' core police powers have always included authority to define criminal law and to protect the health, safety, and welfare of their citizens.
Congress does not share in this power. The powers of Congress are expressly presented in the Constitution and federal drug legislation is not one of those powers unless it relates to interstate commerce. Medical marijuana use under the laws set forth in the handful of states that permit the behavior is limited to home grown pot for personal consumption within the state and state grown pot for personal consumption within the state. This does not sound like interstate commerce to me. What do you think?