I don't know that such medications would be prescribed p.r.n. for a patient because of the chance of overdose? Nailing down when one should/should not take it so they can't take "too much" if they're taking it according to instructions is probably a legal concern and I don't know that insurance companies would like it either if they didn't know when they were going to have to pay for it. I imagine one could take something less than prescribed though, is that sort of what you're asking, if people and their doctors have a separate "yeah, just take it when needed but not more than prescribed" like I have for my Asthma/Albuterol inhaler?