Alcohol and tobacco are more harmful than many illegal drugs including the hallucinogen LSD and the dance drug ecstasy, according to a new scale for assessing the dangers posed by recreational substances.

Drug specialists say the current system for ranking drugs - class A for the most dangerous to class C for the least dangerous, as set out in the Misuse of Drugs Act - is irrational, arbitrary and “lacking in transparency”.

Scientific evidence shows that heroin and cocaine are correctly ranked as class A drugs as they do cause the most harm. But LSD and ecstasy come close to bottom of the league in terms of harm caused, yet they are also labelled as class A.

Alcohol is legal and widely used but comes fifth in the “harm” table, ahead of amphetamines and cannabis, which are ranked as class B and class C respectively. Tobacco is also ranked as more harmful than cannabis.

The league table of 20 drugs drawn up by drugs specialists is intended to provide a scientifically based model for policy makers of the harm they cause. It shows that the dangers they pose bear little relationship to the official classification, on which the penalties for drug use are based. The eight drugs ranked as most dangerous include two that are unclassified while the eight judged least dangerous include two class A drugs.

The report comes a fortnight after an independent commission called for a radical overhaul of Britain’s drug laws which it said were driven by a “moral panic”. The commission, set up by the Royal Society of Arts, said the aim of public policy should be to reduce the harm drugs cause, not send people to jail. It proposed reclassifying drugs - legal and illegal - according to the harm they do.

Professor David Nutt, who works in addiction psychiatry at the University of Bristol and who led the latest research, said: “The current drug classification system is arbitrary in the way it assesses harms. It is not fit for purpose. We have tried to come up with a better system by looking at the factors that contribute to drug use and the harms they cause. We should review the penalties for drug use in the light of the harms they cause and have a more proportionate response.”

Professor Colin Blakemore, chief executive of the Medical Research Council and co-author of the study, said: “The object was to bring a dispassionate approach to a very passionate issue. Some conclusions might appear to be liberal in stance, but that was not our starting position. We intended to reach conclusions that were evidence-based.”

“Alcohol and tobacco are way up there in the league table, not far behind heroin and cocaine and street methadone. Society has not only come to terms with alcohol and tobacco but is well aware of the harms associated with them so we felt it was useful to include them as calibration points for other drugs.”

All drugs were marked on the physical harm they caused to the individual user, their tendency to cause dependence and their social harm, including their effect on families, communities and society [such as crime and NHS costs]. Each was given an overall harm score by two separate groups of experts which yielded roughly similar results.

There was little evidence that ecstasy caused extensive harm, despite its widespread use by young people in clubs and pubs at weekends. Cannabis has been cited as a cause of schizophrenia but the authors said a causal relationship had not been established. If it were, evidence showed no more than 7 per cent of cases could be attributed to use of the drug.

Professor Leslie Iversen, of the University of Oxford, said there was a widespread myth that skunk, from the tips of the cannabis plant, was 20 to 30 times more powerful than that available 30 years ago. “It is simply not true,” he said. “The Advisory Council on the Misuse of Drugs looked at this carefully. Cannabis resin [hash] has changed little and is about 5 per cent tetrahydrocannabinol (THC). Skunk has 10-15 per cent THC. That makes it two to three times more powerful, not 20 to 30 times.”

The study, which took five years to complete, is published today in The Lancet. Professor Blakemore said: “We hope that policy makers will take note of the fact that the resulting ranking of drugs differs substantially from their classification in the Misuse of Drugs Act and that alcohol and tobacco are judged more harmful than many illegal substances.”

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