In my view, perceived harms associated with drugs are vulnerable to so many restrictions on reliability and validity that, for the time being, a serious estimate of drug harm per drug is impossible. In my view, it is even invalid to associate harms to drugs alone. Drugs are used by humans, under individual, social and legal conditions, in certain purities and dosages. Whatever the 'effects' of drugs, harmful or not, they cannot be estimated or even discussed without associating the drug with a particular user or user culture. Drugs per se do not meaningfully exist.
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Without some prior agreement about which set of variables of user characteristics, cultural context and drug dosage and purity is used, even a measure of minimally standardised 'drug harm' cannot be established. Without such prior agreements, serious evaluation of drug harm is an illusion.
Quite probably, the harm ranking that Nutt and colleagues have produced, with the most prevalent drug (alcohol) at number one and one of the least used drugs (heroin) at number two, is a function of popular perceptions among experts. And popular perceptions change and change over time. No amount of sophisticated statistical processing of the combined perceptions of experts can overcome this.