Drug prohibition is like socialism: no matter how badly or how many times it fails, people just keep believing in it.
The other solution, mooted today by police and Iain Dale, of effectively nationalising drug dealerships via the NHS, is socialist and doomed in equal quantities.
My preferred solution is for a Vice Act. Thus, all the main things that humans do that are narcissistic (drugs, prostitution, porn, gamgling) should be treated in the same way: they should be allowed but at the same time clearly marked as not socially constructive. If a person is determined to delve the depths of human depravity then no regulatory force in the world is going to stop him. Instead the role of Government should be limited to ensuring that nobody goes there by accident, that the road to Hell should be clearly signposted.
For example, sex shops and gambling shops should return to being closely controlled with blacked out Windows. They should be forbidden to advertise. Drugs should be sold in the same way and not in a shop that is allowed to sell anything else, especially not another vice, i.e. no mixing drink with gambling or drugs with prostitution.
If drugs were legalised this way, I think the social message that this sent out would be more powerful than prohibitive legislation (as Cameron argues) and drug usage would fall.
As a small businessman myself, I get far more hassle from the authorities than the average drug pusher. I can even go to prison now for not putting my company registration at the bottom of an email. It would be nice to see such bureaucracy and the Health & Safety gestapo decending on the drug dealers, again with a likely decrease in deaths as well as overall turnover.
Finally, I think it's worth noting that drug prohibition is at the root of our problem with Islamic terrorists. If drugs were legal, then bandit states like Colombia and Afghanistan would never have got so lawless or, ironically, so impoverished - and in turn have created ideal breeding grounds for extremists.