It was a Tory politician's soundbite that prompted the Scottish poet and novelist Kevin Williamson to go from writer to campaigner.

The former Scottish secretary Michael Forsyth referred to drug users as a medieval plague in our midst. Williamson saw red, and his book, Drugs and the Party Line, is the result.

"Most of the people I know who use drugs don't abuse them," he says. His book argues that no matter how much money is thrown at the drugs war, demand will continue and people will supply it.

This afternoon he is hoping for a bit of hostility when he hosts a debate in Dublin on the legalisation of drugs. His "Change the Drug Laws" roadshow has visited London, Cardiff, Belfast and Amsterdam, where he felt he was preaching to the converted.

The audiences have been overwhelmingly young and unanimous in their frustration with the status quo. "We have yet to find one person who agrees with the drugs laws," he says. His book is not pro-drugs, but anti-prohibition, he maintains.

Coming from a small Edinburgh publisher under Williamson's own imprint, Rebel Inc, the book is aimed at people who are more likely to buy albums than novels. The target audience is the same group that is turned off by official anti-drugs campaigns.

Rebel Inc started life as a magazine of new writing published by Williamson in the early 1990s aimed at the clubbing generation. One of its most notorious issues contained an article in which Trainspotting author Irvine Welsh introduced Williamson to ecstasy and the two conducted an interview under the influence.

Welsh has written the introduction to Drugs and the Party Line, and the author and convicted cannabis dealer, Howard Marks, is involved in the tour. The book comes in two versions, one printed on ordinary paper, the other (£1 more expensive) on cannabis hemp.

Williamson has been pleasantly surprised by the turnout at public meetings. "We thought it might just be a load of old sorta hippies, but it's been a lot of young working-class people who are really interested in debating the issues, rather than stoners going on about the joys of cannabis."

He has also been surprised by the strength of feeling and the horror stories. British MP Paul Flynn, one of two Labour MPs who has publicly backed the campaign, told a meeting in Cardiff that police were no longer prosecuting people caught with small amounts of cannabis.

However, a man in the audience said he had been caught with just one gram of cannabis, worth about £10 or £20. "He was fined £125. He lost his job as a teacher and when he was unemployed he got involved in all sorts of stuff. Now he's on a heroin treatment programme."

Williamson argues that the drug laws are doing more damage than the drugs themselves. And the "war on drugs" approach has stifled debate about whether that method is working, or can ever work.

"There have never been public meetings across the country when people can come along and discuss the drug laws. Politicians refuse to discuss it, and the drug debate outside parliament is intense."

Williamson argues that cannabis should be legalised and if that works the other drugs should follow. "If you only decriminalise possession, that leaves supply in the hands of the same criminal element."

But legalisation of supply does not mean that cannabis becomes as freely available as tobacco, he says.

"It makes sense to have any drugs for sale, whether cannabis, tobacco or alcohol, strictly licensed." The licensing would be similar to the drinks licensing system where strength, times and location of sales and age limits were strictly laid down.

"It's a myth that somebody who takes cannabis is going to want to go on to harder drugs to get a better hit. The majority of heroin users are people who've abused alcohol. What can't be denied is that the same people who are selling cannabis are selling heroin."

The only people that benefit from the drugs black market are "criminals and drug dealers and the anti-drugs professionals," he says. Cannabis accounts for 80 per cent of the drugs market in Britain.

Williamson would like to see heroin made available on prescription for addicts. An experiment in one British town in the early 1990s had "spectacular results in tackling crime," he said. There was a 93 per cent drop in property crime when addicts were prescribed heroin. The heroin-substitute methadone merely resulted in "too many addicts topping up the methadone with heroin."

The British media are divided on the issue. The tabloids "continue to hammer out the same line," he says: "Crack down on dealers and just say no." The majority of the broadsheets support a different approach. The book was also prompted by a government drugs initiative called Scotland Against Drugs. With more than a little relish Williamson refers to it as SAD throughout his book.

"Everyone in Scotland just referred to them as sad bastards. They were completely isolated from their target audience."

One of the advertisements was a bus poster which showed a prison and said: "There's plenty more room for dealers inside". However, Scottish prisons, like their Irish counterparts, are chronically overcrowded.

Ecstasy is one of the most dangerous drugs to leave in the hands of criminal dealers, Williamson argues. "People are not getting the proper advice on how to use it and people have died."

He is hopeful that the British government will listen to the argument. "I'm not hopeful it's going to happen tomorrow. But it does seem inevitable. It's almost like a volcano erupting in Britain and it's different from the last time round when people campaigned.

"In the Sixties it was all about freedom to do anything you wanted. Now a huge criminal cartel is controlling drugs and the mayhem and violence associated with it now are destroying whole areas of the country." A change in the laws would benefit people who do not use drugs, he says.

Between three and seven million people in Britain use illegal drugs, he says. The figures are hazy because of the black market nature of the supply. "Out of that number there are between 150,000 and 200,000 addicts. So around 3 per cent of illicit drug users are actual drug addicts."

The US authorities spend $15 billion a year on the "war against drugs. In Britain you just can't get the figures."

That money could be spent on building decent amenities for the people who use drugs to "escape" from reality. Drugs should be a health and social issue, he says, rather than a criminal issue.

And what about the argument that Britain would become a drug destination? "In the long run it would be unrealistic to break with the European drug policy and Britain would end up with drug tourism." But, he says, pressure is growing in countries like Denmark and Germany to legalise cannabis.

The debate chaired by Kevin Williamson and Hot Press journalist Olaf Tyranson takes place today at 3.30 p.m. at the Temple Bar Music Centre in Dublin. Entrance: £2