I seem to remember that CMS was threatening providers that prescribed anything for covid not recommended by the CDC.
They “recommended” and “suggested”.
Since it wasn’t a covered countermeasure, however, it was all for show.
State Licensing Boards have very broad authority to regulate the practice of medicine, much broader than the FDA. The CDC has no authority over medical practice.
Although you can prescribe any (licensed) drug for any purpose you believe it to be useful for, you can’t do wrong things.
If you believe Nexium treats breast cancer, and 20 patients you give it to all die from breast cancer, you are going to have a problem with your license.
But the Boards have to play fair. “Respectable minority” opinion is usually enough to make them happy.