The resulting question, of course, is whether Rogaine is a medication for medical purposes. No one at the checkpoint will be remotely qualified to make that assessment, but that doesn't mean they wouldn't try.
Sometimes bureaucracy is great: the FDA has already declared Rogaine to be a legal drug; i.e. it is for medical purposes. Using it for any other purpose would be illegal (I think).
A TSO declaring that Rogaine is not a medication for medical purposes is trying to overrule the FDA. Either that or any TSO could accuse anyone carrying any medication (liquid or not) of breaking the law; i.e. "prove that you intend to take that APAP (Tylenol) as directed. You can't? I NEED A REAL COP NOW!".
