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.
They are a poison.
I don't think you get it. Everything is poison in the appropriate dose. What usage is "as directed" is irrelevant if you can freely acquire the necessary amounts.
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.