I mean, sure, its addictive and unhealthy. But, the people suffering from it are the people that take it in the first place, so, banning personal possession and use is effectively punishing the victims, adding more suffering which may drive some to seek more drugs to get through. And if you instead ban production and distribution, you simply create a perfect niche for organized crime to appear to do those things, which creates worse problems than the drugs themselves. If such drugs dont force those involved with them to hide, at least it is possible to put some regulations on them, and more easily monitor and intervene in their use.