Your body generally prevents you from drinking too much water, and from eating too much food, etc. Besides, water and food is a necessity while heroin is not (far from it, almost no one will ever need it).

And that's what the person above meant: it's not the highly addictive or the easily lethal nature of heroin that is a problem, but the combination of both. Most things that combine these two attributes are illegal.

However, I go agree that there is a problem with the fact that it is so addictive that making it illegal makes things worse because of low quality products. Doesn't undermine the point of the post above though. In an ideal world, no one would try to get heroin.