Recently, David Nutt , Leslie King and Lawrence Philips published an article in The Lancet (Nutt et al, 2010) on how to rank drug harms between 20 different drugs available in the UK. They describe a procedure by which an unknown number of drug experts rank these drugs on 16 criteria on a scale of 0 to 100. According to this decision-making process, the most dangerous drug is alcohol with a score of 72 (out of 100), followed by heroin (score 55), crack cocaine (score 54) and methamphetamine (score 33). On their scale, a drug that was perceived as extremely harmful and even neurotoxic two decades ago gets a score of no more than 9 (ecstasy).
The inference that drugs cause harm is so accepted that few would even think of asking questions about how we know this. Still, the answer is not easy to give, since many users of all the ranked drugs will never suffer any harm. It is like participating in heavy urban traffic: some participants will get wounded or even killed, but most will never suffer any harm or other negative consequences.
I would like to comment on the ranking of drugs in terms of their harmfulness that Nutt and colleagues propose, on the basis of a particular expert-driven decision-making procedure. The question is, of course, whether drug harm can be estimated and ranked in a valid way by experts, without any quantifiable and quantified definition of harm, or indices of risk. My answer to this question is 'no' and in the following text I want to explain why. [Extract]