David Nutt, a professor of neuropsychopharmacology at Imperial College London and a former chief adviser on drugs to the British government, said the international prohibition of psychedelics and other mind-altering drugs over the past half century has had damaging and "perverse" consequences.

"When a drug becomes illegal, conducting experimental research on it becomes almost impossible," Nutt told reporters at a briefing in London ahead of the publication of his new book "Drugs - without the hot air".

He compared the situation with that in stem cell research under former U.S. President George W. Bush, who banned any new embryonic stem cell studies from 2001 to 2009 - a move many scientists consider held the field back for years.

Nutt said the problem with the current approach to drugs policy globally, which is centered on the banning of substances thought to be most harmful, "is that we lose sight of the fact that these drugs may well give us insights into areas of science which need to be explored and they also may give us new opportunities for treatment."

"Almost all the drugs which are of interest in terms of brain phenomena like consciousness, perception, mood, psychosis - drugs like psychedelics, ketamine, cannabis, magic mushrooms, MDMA - are currently illegal. So there's almost no (scientific)work in this field," Nutt said.

Nutt last year conducted a small human trial to study the effects of psilocybin, the active ingredient in magic mushrooms, on the brain.

Contrary to scientists' expectations, the study found psilocybin doesn't increase but rather suppresses activity in areas of the brain linked to depression, suggesting the drug might be a useful treatment for the debilitating condition.

Nutt said he was forced to "jump through hundreds of hoops" to be able to conduct the study, having to comply with a level of complex, expensive and time-consuming security and regulation that would put most scientists off.

WHAT DRUGS ARE AND WHAT THEY DO

The professor, who was sacked in 2009 in a high-profile row with the British government after he compared the risks of smoking cannabis with those of riding a horse, said he was driven to write the book in the hope of improving understanding of drugs - both legal and illegal, medicinal and recreational.

"There is almost no one in society who doesn't take drugs of some sort. The choices you make in your drug-taking are driven by a complex mixture of fashion, habit, availability and advertising," he said.

"If we understand drugs more, and have a more rational approach to them, we will actually end up knowing more about how to deal with drug harms."

Published on Thursday, the book seeks to explore the science of what a drug is and how it works.

It discusses whether the "war on drugs" did more harm than good - Nutt thinks it did.

And it explores why Britain's Queen Victoria took cannabis - apparently her physician J.R. Reynolds wrote a paper in the Lancet medical journal saying that "when pure and administered carefully, it (cannabis) is one of the most valuable medicines we possess". He prescribed it to the monarch to help her with period pains and after childbirth.

The book also has chapters on why people take drugs now, how harmful they are, where and whether the danger lines should be drawn between legal drugs like tobacco and alcohol, and illegal ones like cannabis and magic mushrooms.

Nutt doesn't dispute that drugs are harmful, but he takes issue with what he says are un-scientific decisions to ban one, like cannabis, while allowing another, like alcohol, to be freely and cheaply available on supermarket shelves.

"Drugs are drugs. They may differ in terms of their brain effects, but fundamentally they are all psychotropic agents," he said. "And it's arbitrary whether we choose to keep alcohol legal and ban cannabis, or make tobacco legal and ban ecstasy. Those are not scientific decisions they are political, moral and maybe even religious decisions."

(Reporting by Kate Kelland, editing by Paul Casciato)

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