The former government drugs adviser forced to resign over his views on cannabis has repeated his calls for the drug to be decriminalised.
Professor David Nutt said a decision to make magic mushrooms Class A was "almost the final nail in the coffin of the rationality of the Misuse of Drugs Act" and there needs to be more "sensible, rational" regulation of drugs.
He said Government decisions on drugs were often motivated by politics rather than science, and stood by previous, controversial comments in which he declared taking ecstasy was no more risky than horse-riding.
Prof Nutt resigned as the chairman of the Advisory Council on Misuse of Drugs (ACMD) in November 2009 over the decision to reclassify cannabis from a Class C to a Class B drug.
Now chairman of the Independent Scientific Committee on Drugs, he told the Home Affairs Select Committee today: "I think people have a very exaggerated perception of the harms of drugs and they tend to minimise the harms of other activities which particularly young people engage in which are potentially as harmful or more harmful."
Prof Nutt called for a system similar to the Bank of England model, whereby the ACMD could have some statutory powers "to stop party politics contaminating sensible decision-making".
"The reason the Misuse of Drugs Act was set up in the first place was to stop people playing politics with drugs," he said.
But he said in his 10 years on the ACMD, politicians would "only support recommendations which made drugs more illegal or increased the sanctions" and said in that time, cannabis was the only drug reduced in classification.
"Moving magic mushrooms to Class A was almost the final nail in the coffin of the rationality of the Misuse of Drugs Act," he said.
"Politics determined decision-making much more than science.
"It's easy to score political points around drugs and that's why we have ratcheted up sanctions, classes, over the last 40 years, and people have not had the courage to say, 'no, it's wrong'."
Prof Nutt told the committee that he believed 25% of the British public would smoke cannabis instead of drinking alcohol if the drug was not illegal, and suggested a system of decriminalising it similar to that used in the Netherlands.
"They are all very rational approaches and they would reduce harm in society.
"What we see now is a rising, rising tide of damage from alcohol. There's no doubt that a lot of people drink because it's legal. It's considerably more dangerous than cannabis."
He said decriminalising cannabis would bring a "net benefit" to the population.
"Of course cannabis is harmful, all drugs are harmful. You can't have a harm-free drug, of course cannabis is harmful, but you have to be proportionate.
"The harms of cannabis are less than the harms of alcohol. Cannabis is not safe but I am saying in proportionate terms that kind of regulation would have a net population benefit on health."
The professor also stood by his comments about horse-riding, saying: "I don't think it's irresponsible, it's entirely appropriate.
"These are all activities that people do because they enjoy doing them.
"It's completely arbitrary you should say you should allow someone to ride a horse and not worry about the cost to the NHS when they fall off and break their brain.
"I think it's completely appropriate to say and, in a broader sense, people want to make a decision about what to do with their life, they should at least know about the harms of drugs as compared to all the other activities that they do."
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