Higher doses of morphium aren't given per pill but intravenously.
As far as I understand in Berlin where I live a patient who's in the hospital and in pain usually get's more morphium if he asks for it.
From a medical perspective it's much easier to dose morphium than to dose marijuana smoking.
Over at Scott Adams' Blog you can find a very fine example of using the 'Rationality Engine' to solve the social problem of assisted dying.