Not too many problems, that's in general exactly how it works today. FDA approvals are generally marketing approvals: what you're allowed to advertise your drug

Not too many problems, that's in general exactly how it works today. FDA approvals are generally marketing approvals: what you're allowed to advertise your drug for and to who. Physicians can do more or less whatever they want in terms of prescribing things. (Drugs which aren't approved for marketing for anything yet are a little different, but can still be made available as an "investigational" drug, which is a much lower, primarily safety, bar.)