"Understandably, it is in everyone's interest that every effort is made to prevent these circumstances arising and I'd be the first to agree that the simplest method is to ban illicit possession of Class B drugs in order to prevent the potentially horrendous consequences of a Class A addict. "
You're kidding right? I've smoked pot for 30 years and never done heroin, despise cocaine, and deplore meth. THC isn't addictive, you can't od on it, and tolerance levels go away in less than three weeks. I think if you showed alcohol consumption as an indicator of future drug use the incident rate of class A abuse would be 100%.