I downvoted you because Viliam claimed medical marijuana prevents a great deal of pain, and you responded with - an argument that fails to even assert it causes pain elsewhere. Perhaps you could start by showing that what you call the "central" use of medical marijuana does not displace other recreational drugs.
He didn't just claim that medical marijuana reduces pain, he claimed that it reduces pain, and therefore people who oppose euthanasia should support it. This argument implicitly assumes, not just that medical marijuana prevents pain, but also that there are no other reasons (including non-pain related reasons) which outweigh the benefit from preventing pain.
In other words, given his argument, any argument that medical marijuana is in general bad--including claims that it is misused for non-medical purposes--is responsive. So is any argument that asserts ...
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.