The current drugs classification doesn't reflect either the harm of social effects of a drug; on the one hand we have alcohol, responsible for an enormous amount of violence, crime, and illness. It is the direct cause of thousands of deaths per year. It is completely legal.
There are then prison sentences for the possession of MDMA and cannabis, despite presenting lower risks to people around the user (note the lack of agression and recklessness characteristic of alcohol). There is no justification for criminalising possession of these drugs for personal use, when they have comparatively small harm to the user.
I believe that the government has, for too long, suppressed any real debate over this matter. This surfaced very publicly with the sacking of Prof. David Nutt, who, as far as I can see, was removed for simply speaking the truth.
The current classification system is incredibly flawed; it simply does not reflect the risk involved. How are the public to make an educated decision if they have no real idea of just how harmful a drug is likely to be? The sad reality of our drugs classification system is highlighted in this graph;
This is is admittedly a couple of years out of date (note that cannabis is a class C), but I've used it simply because of how straightforward and clear it is. A slightly more in depth look at the harm of each substance can be found here;
Source; Lancet. 2007 Mar 24;369(9566):1047-53.
It is very clear that the classification of a drug simply isn't directly related to its harm. This means that from the information presented in law by the government, your average citizen doesn't know the difference in the risk between taking Cocaine and MDMA, as they are both a Class A drug. This also means that after taking one of the less harmful Class A or B drugs, trying another might seem like a reasonable idea.
People need to know that a drug is classified in the way it is for a reason, thus avoiding the more harmful ones with harsher penalties. Without this, confidence in the system will forever be lost, and an effective deterrent can't exist in law.