Morphium is also capable as a painkiller. I'm not aware that there pain for which marijuana is the only possible way to stop the pain.
It may depend on country, but in situations I have seen, the patients were not able to receive enough morphium to stop the pain most of the time. There were limits on how often they can get a pill, and if I remember correctly, it was once in a few days, but the pill only worked for a few hours.
The limits on how much morphium one can get are probably motivated by trying to (a) minimize the risk of addiction, and (b) avoid health damage. Both of that is nonsense if we speak about a person who obviously only has a few weeks of life left and zero chance to rec...
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.