The current federal policy on marijuana has been to not interfere with the states that have legalized it within their jurisdiction. However, the upcoming Attorney General, Jeff Sessions, has long expressed views that would indicate that he intends to overturn this policy.
If it happens, states that maintain databases of medical marijuana patients can be significantly affected, as those databases can then be used by the federal government as ready-made lists of people to prosecute for violation of the federal law.
In Controlled Substances Act, Congress gave the executive branch authority to reschedule or remove substances at its discretion. President Obama should exercise that authority, via an Executive Order, to protect vulnerable people against prosecution, and let the states decide.