A deliberately lethal medicine is simply a poison.
The dose makes the poison and that has been known for ages. You can kill a man with water.
"Euthanasia drugs" only make death pleasant and painless. Often they are just an overdose of opiates.
They are a poison. The fact that they can do other things is irrelevant if you're using them as directed for a poison. Although I suppose there might be problems if someone claims to be suicidal so they can underdose and use it for opiates.
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.