I heard something on the news about this a week or two ago, but was away travelling so didn't get round to posting. It seems that the government are banning currently legal drugs such as Mephedrone.
I heard people saying that, because they're dangerous, they ought to be banned. Lots of things are dangerous but not banned, like extreme sports or an unhealthy lifestyle. No doubt John Stuart Mill would be turning in his grave. His famous 'harm principle' insists that we should only interfere with someone's liberty in order to prevent harm to others, not for their own good.
If a drug is dangerous, then the appropriate thing to do (as we do with tobacco, for example) is to warn people, and maybe regulate it (obviously we need to be sure that it doesn't end up in the hands of children, etc). To ban something just because we think that people would be better off not doing it risks the state imposing too far on individuality. Next we know the state may be banning a whole range of activities on the grounds that they're bad for those who indulge in them...