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 that pain relief is relatively infrequent, because he is (implicltly) balancing pain relief against other reasons, and the other reasons affect the result more if pain relief is infrequent. Just because his claim is that it reduces pain doesn't mean that counter-arguments must be limited to asserting that it causes more pain elsewhere.
Viliam:
I got the impression that the anti-euthanasia guy is mostly repeating one argument -- that you can always remove pain, therefore the argument of killing people to prevent them from feeling unnecessary pain is always false.
...From what I have seen, hospitals for terminally ill patients are more or less torture chambers.
But sure, if you had argued that we should view marijuana use as approaching torture in badness, or even that the opposition takes this view, that would be responsive.
Now the Catholic Church does in fact seem to oppose marijuana ac...
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.