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Scientists devise ranking table for drugs

March 23, 2007 By Michael Hopkin This article courtesy of Nature News.

Revised league table could reopen debate on drug classification.

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British scientists asked by the government to rank the harm of different drugs of abuse today publish their results in the Lancet1.

The new system, which puts alcohol and tobacco below heroin but above cannabis, is an attempt to provide a scientific — if still simplistic — way to compare the social and health tolls taken by recreational drugs.

"The current drug classification system is rather arbitrary in terms of the way it assesses harm," says David Nutt of the University of Bristol, UK, and one of the team who devised the new system. Current British drug laws, he says, are shaped by political prejudice as much as by the actual threats posed by the substances.

Recreational drugs pose various types of threat — from the possibility of an accidental overdose causing sudden death, to a parent's desire for alcohol pulling them away from their responsibilities at home. Comparing such physical and social harms, for both long-term and short-term effects, is very difficult.

Nutt and his colleagues used a simple system to approach the problem. They set up three categories of threat — physical harm, dependence, and social harm — and divided each of these into three sub-categories (see 'The categories of harm').

They then asked experts - including psychiatrists specializing in addiction, members of police, forensic experts, chemists and doctors - to give up to 20 drugs a score out of three for each of the nine categories. The average scores for each of the three main categories of threat were then simply added together and averaged again to calculate an overall score out of three.

The result? Heroin and cocaine were ranked as the most dangerous, reflecting their status as class A drugs — the most harmful tier of Britain's three-category system, and those that carry the stiffest legal penalties. But ecstasy, another class A drug, finished eighteenth in their list — below commercial solvents and anabolic steroids. Alcohol was ranked fifth most dangerous, and tobacco ninth.

Mix and match

There are difficulties with this system, the authors admit. The effects of some drugs depend heavily on their formulation. Crack cocaine is generally deemed more dangerous than the powdered version of the drug, for example, but the researchers did not consider the two formulations separately. And use of more than one drug at the same time can muddle up the results. Cannabis is usually smoked with tobacco, they note, which might have inflated its score for physical harm.

The system also weighs equally the damage done by physical or social harm and dependence. The authors admit that this approach isn't always appropriate. But the full table of their results could be consulted if one was more interested in one type of harm over another (see 'How some common drugs stack up').

Robert MacCoun, a specialist in drug use and policy at the University of California, Berkeley, explains that although it's a worthwhile exercise to attempt to rank the harm of drugs, it isn't always possible. "The exercise is fraught with peril," he says. Squashing several evaluations into a single number is not a good idea, he adds. "It's best not to force things onto a single dimension."

Several other attempts to rank drugs, including some by the World Health Organization, ran into political problems and so did not amount to much, he says.

Open issue

The findings demonstrate the need to reopen the issue of how drugs are classified, says Nutt. "The principal objective of this study was to try to bring a dispassionate approach to what is otherwise a very passionate issue," says Colin Blakemore from the Medical Research Council in London, who also worked on the study. "We want a good system for explaining harm."

Nutt and his colleagues are due to report their recommendations to the UK parliament later this year.

The researchers are aware that the social use of alcohol and tobacco are now too entrenched to be changed, and that there are complicating factors involved in making drug policies — such as the effect that legalization has on availability or purity of a drug. But the researchers hope that their findings will stimulate debate, and prompt other countries to look at drug harm in a similar way. "No other country has a framework like this," says Blakemore.

Additional reporting by John Whitfield


  1. Nutt D., King L. A., Saulsbury W. & Blakemore C. Lancet, 369 . 1047 - 1053 (2007).


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