As the headlines this week alone demonstrate, the whole process of determining drug classification has become quite complex and highly politicised. I focus on cannabis partly because it is the only drug that has been downgraded in the whole history of the 1971 Misuse of Drugs Act, which established the present system of drug classification, but also because the issues relating to cannabis pose a challenge to whether the act is working as it was originally intended.

The Advisory Council on the Misuse of Drugs (ACMD) was requested by the home secretary in 2007 to review the status of cannabis because: "Though statistics show that cannabis use has fallen significantly, there is real public concern about the potential mental health effects of cannabis use, in particular the use of stronger forms of the drug, commonly known as skunk."

So, there was a skunk scare. Cannabis had gone from class B to C, but, supposedly, skunk use had been increasing and it was getting stronger, so we were asked to review whether the decision to go from B to C was still appropriate. In what was the ACMD's third cannabis report (Rawlins et al, 2008), we came to several conclusions:

● Cannabis is a harmful drug and there are concerns about the widespread use of cannabis among young people.

● A concerted public health response is required to drastically reduce its use.

● Current evidence suggests a probable, but weak, causal link between psychotic illness and cannabis use.

● The harms caused by cannabis are not considered to be as serious as drugs in class B and therefore it should remain a class C drug.

On that final point, there has been a lot of commentary and some research as to whether cannabis is associated with schizophrenia, and the results are really quite difficult to interpret.

What we can say is that cannabis use is associated with an increased experience of psychotic disorders. That is quite a complicated thing to disentangle because, of course, the reason people take cannabis is that it produces a change in their mental state. These changes are a bit akin to being psychotic – they include distortions of perception, especially in visual and auditory perception, as well as in the way one thinks. So it can be quite hard to know whether, when you analyse the incidence of psychotic disorders with cannabis, you are simply looking at the acute effects of cannabis, as opposed to some consequence of cannabis use.

If we look on the generous side, there is a likelihood that taking cannabis, particularly if you use a lot of it, will make you more prone to having psychotic experiences. That includes schizophrenia. But schizophrenia is a relatively rare condition, so it's very hard to be sure about its causation. The analysis we came up with was that smokers of cannabis are about 2.6 times more likely to have a psychotic-like experience than non-smokers. To put that figure in proportion, you are 20 times more likely to get lung cancer if you smoke tobacco than if you don't.

The other paradox is that schizophrenia seems to be disappearing (from the general population), even though cannabis use has increased markedly in the last 30 years. So, even though skunk has been around now for 10 years, there has been no upswing in schizophrenia. In fact, where people have looked, they haven't found any evidence linking cannabis use in a population and schizophrenia.

Nevertheless, one of the key arguments in moving cannabis from class C to B was the concern that skunk would cause more psychosis. What is very regularly invoked in this debate is the precautionary principle, which is that, if you're not sure about a drug harm, rank it high, make all drugs class A and get rid of the problem.

In her statement to the Commons, after receiving the ACMD's report but rejecting its recommendation to keep cannabis as a class C drug, Jacqui Smith, the former home secretary, said: "We must err on the side of caution and protect the public." As this is protection from the known unknowns, at first sight it might seem the obvious decision – why wouldn't you take the precautionary principle?

But the precautionary principle is also an act of faith in deterrence, and this is one of the key issues for lawyers. Does deterrence impact on drug use? We don't know. In fact, the outcome may be the opposite of that predicted. It may be that if you move a drug up a class, it has a greater cachet.

I think we have to accept young people like to experiment, and what we should be doing is to protect them from harm at this stage of their lives. We therefore have to provide more accurate and credible information. We have to tell them the truth, so that they use us as their preferred source of information. If you think that scaring kids will stop them using, you're probably wrong.

This article is based on a longer version published by the Centre for Crime and Justice Studies at Kings College, London, which itself was based on a lecture delivered by Professor Nutt earlier in the year.