"Queen Victoria wrote eulogies about cannabis," says David Nutt. "She used it for period pains and post-childbirth - and a hundred years on we lock up people with multiple sclerosis who use it."

A similar scepticism about government policy is a typically prominent feature of his new book, Drugs - Without the Hot Air: Minimizing the Harms of Legal and Illegal Drugs, published on 31 May.

Although he is the Edmond J. Safra professor of neuropsychopharmacology at Imperial College London and a distinguished researcher into the mechanisms underlying addiction, Professor Nutt knows that he will always be most famous as "the scientist who got sacked" from his post as chairman of the Advisory Council on the Misuse of Drugs (ACMD).

He had already had his knuckles rapped in February 2009 by Jacqui Smith, who was then home secretary, for an article in which he compared the relative dangers of ecstasy and horse riding.

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Then her successor Alan Johnson dismissed him in October of that year for giving a lecture in which he argued that it made no sense to put drugs such as cannabis into a higher legal class than the evidence justified.

Free after that to be as outspoken as he wished, Professor Nutt was soon invited to help set up the Independent Scientific Committee on Drugs, where he was joined by many of the experts who resigned from the ACMD in protest at his dismissal.

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His book, he tells Times ÖйúAƬ, draws on "personal experience with patients [as a psychiatrist] and politicians, while treating each drug dispassionately, trying to be scientific about it".

Professor Nutt's clinic deals mostly with people who need medication for problems such as attention deficit hyperactivity disorder and insomnia. "Doctors are often frightened of prescribing amphetamines for ADHD," he says. "That is partly why I wrote this book, so I can set out the real risks and benefits."

We already understand a lot about drugs, according to Professor Nutt. "Alcohol and tobacco are no different from other drugs, so the separation between legal and illegal is arbitrary in terms of biology and science. It's either political or moral, or a combination of the two."

It is this combination, in his view, that has led to many absurdities. "The Drugs Act 2005 tells schoolchildren that magic mushrooms are as dangerous as crack cocaine," he explains.

"Even eight-year-olds know that's wrong. You can't have a dialogue about harms if what you are telling people is so blatantly wrong."

Professor Nutt points out that fresh magic mushrooms were legal until 2005, "when the Labour government wanted to demonstrate [it was] harder on drugs than the Tories".

He would like to see the attitude towards drugs change from a penal to a health-focused approach. Professor Nutt is highly critical of the "war on drugs" and believes we need "a new drugs act, so there's a proportionate harm-based response, while there are also policies to reduce the harms of alcohol".

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He insists that this is where the UK authorities should be focusing their attention, as the problems associated with alcohol are "by far the greatest expense to society".

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He even wonders whether drugs that are "less harmful to individuals than alcohol" should not be available "under some kind of regulated access". After all, he asks: "Why should you be forced to use alcohol to intoxicate yourself?"

However, he does concede that in the case of more dangerous drugs, he would prefer to educate people not to use them.

"In the Netherlands, the licensed cafes create a separation of markets, so people who want to use cannabis don't need to go to heroin dealers," he argues.

Healthy dissent

Still convinced that "the scientific collection of evidence should direct policy", Professor Nutt remains unimpressed by the attempts of most politicians to get to grips with drugs.

"Gordon Brown is the son of a Scottish minister, and what they believe in is alcohol. Whisky is the defining drug of Scotland, so his attitude was: whisky good, everything else bad."

Ms Smith, in his view, was no better. "She introduced a draconian policy of arresting kids on the street with cannabis, despite the fact that she, as a student, had been a smoker - despite knowing that if she'd been arrested, she would have had no political career."

Today, however, Professor Nutt notes the current prime minister's "very interesting drug-using career at school" and sees distinct possibilities in "the Cameron approach".

Referring to an incident revealed in 2007 in a biography on the Tory leader, he says: "When Cameron was caught at Eton in possession of cannabis, he was given hundreds of lines of Latin to copy out.

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"Perfect. Let's give every schoolboy 500 lines for smoking cannabis rather than kick them into the street so they start selling the stuff or start selling heroin. That was a sensible approach which allowed his career to blossom."

matthew.reisz@tsleducation.com.