There are lots of drugs people can take without doctor's approval. It's just ones with harmful side-effects that are a problem. Drugs for euthanasia are, for all intents and purposes, 100% effective with no side-effects.
Drugs for euthanasia are sugar pills with 100% occurrence of lethal side effects.
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.