A model already exists in work done by the Advisory Council on the Misuse of Drugs itself for the construction of such a harms index. An unofficial matrix of drug-related harms is already the informal mechanism on the basis of which the ACMD’s Technical Committee has made some of its recommendations. Professors Blakemore and Nutt have worked with William Saulsbury of the Police Foundation and Leslie King of the Forensic Science Service on what David Nutt has described as ‘a matrix in which numerical values could be given to assessments of harm in order to rank drugs, not just illegal drugs but also including the familiar, acceptable, legal drugs as a kind of calibrator for the scale as a whole’




The index, a simple list of substances set out in descending order of harmfulness, could be generated by a matrix mapping the various types and degrees of harm associated with each of the substances in question. These harms would be related to, among other things, the substances’ chemical properties. Thus it would be possible to differentiate, say, between different grades of cannabis according to their levels of THC or tetrahydrocannabinol, allowing skunk and the many other varieties of high-strength cannabis to be ranked higher than the most common street-level varieties.

But the catalogue of harms would not be restricted to the substances’ chemical characteristics. It would also—and this is critically important—incorporate the context, the circumstances and the ways in which the substances might be used, for example:
 
(a) the effects that a substance may have on people with particular characteristics—high blood pressure, for example, or a tendency to depression;

(b) the risks inherent in particular methods of taking substances—injecting crack, say, rather than chewing coca leaves, combining heroin with alcohol, snorting ketamine, injecting in the groin;

(c) the links between individual substances and particular types of crime—between crack and violent behaviour, heroin or cocaine and shop-lifting, alcohol and domestic violence, GHB and (allegedly) date-rape;

(d) the propensity for some substances to be used in binges; and so on.

The matrix would set out each of the categories of harm that substances may cause, and each individual substance would then be scored in each category. Its mean score would be used, but only as a guide, to help determine its relative position on the harms index.

The index should be based on the best available evidence and should be able to be modified in the light of new evidence – and also in the light of the coming onto the market of new substances.

It should be intelligible to lay persons as well as to scientists and lawyers, and the evidence on which it is based should be made readily available to the public.

There should be no mystery about it, as there is about the present ABC classification, which often appears to be arbitrary, confused and haphazard.