Why are we asking this now?

Because yesterday a British think-tank published a report for next year's United Nations Strategic Drug Policy Review, suggesting that a decriminalised, regulated market in cannabis would cause less harm than the prohibition of the drug currently in force across most of the world.

What is the UN review?

It is an examination of progress made since the international community, at a special session of the UN General Assembly in New York in June 1998, agreed a 10-year programme of activity for the control of illegal drug use and markets – the "war on drugs". It is thought unlikely that enormous progress will be reported in 2009, as many drugs are purer, cheaper, and more widely available than ever before. Experts on drug policy are therefore looking again at the alternative to prohibition which is always in the background, but which no office-holding politician hoping for re-election appears able to contemplate - legalisation.

What exactly is the think-tank report?

It is the Global Cannabis Commission report, launched at a conference in the House of Lords yesterday and prepared for the Beckley Foundation, a charitable trust "set up to promote the investigation of consciousness and its altered states from the perspectives of science, health, politics and history." The report, put together by a specially-commissioned international group of academics and experts in drug policy analysis, attempts to put the issue of cannabis in a global perspective with a comprehensive view of the evidence, so that governments can move beyond what is termed "the present stalemate in cannabis policy."

Which stalemate is this?

Cannabis is used worldwide by "a conservatively estimated 160m people", according to the report, so it can hardly be said that prohibiting it is successful – and increasingly, nations cannot agree on the way forward. Some countries take a hard line – in the US, about three-quarters of a million citizens are arrested every year for cannabis possession – while other countries have considerably relaxed their penalties or their enforcement policies (Until recently Britain could have been put into this category. Four years ago we downgraded dope from a class B to a class C drug – until in May, the Home Office, clearly at Gordon Brown's behest and in the face of official advice to the contrary, retightened the policy and made it class B once again, after fears in some quarters that stronger versions of the drug were leading to more harmful effects.) But internationally, cannabis is considered an outlawed substance, so changing the official regime is everywhere difficult.

Why does the report suggest cannabis should be legalised?

It argues that although cannabis can have a damaging effect in health and on mental health, it is actually far less damaging than alcohol and tobacco. "Historically, there have only been two deaths worldwide attributed to cannabis, whereas alcohol and tobacco together are responsible for an estimated 150,000 deaths per annum in the UK alone," the report alleges.

Much of the harm associated with cannabis use is "the result of prohibition itself, particularly the social harms arising from arrest and imprisonment," the report says, claiming that policies which control cannabis, whether draconian or liberal, appear to have little impact on the prevalence of consumption. It offers the alternative of a legal but properly regulated market.

"In an alternative system of regulated availability, market controls such as taxation, minimum age requirements, labelling and potency limits are available to minimise the harms associated with cannabis use," it says, claiming that through a regulated market young people could be protected from the increasingly potent forms of the drug, such as skunk.

Wouldn't the legalisation of cannabis pave the way to the legalisation of all drugs?

It might well do, which is why, no matter what the relative harm of dope may be compared to cigarettes or whisky, a move to end prohibition would be stoutly resisted by opponents of liberalising the drug laws, and welcomed by those who would like to see liberalisation brought in. For it is the issue of prohibition itself, rather than the issue of cannabis, which is really at the heart of the argument. The drugs-liberalisation pressure group Transform yesterday welcomed the Global Cannabis Commission's call for legalisation, but said it would also welcome its now being applied to heroin and cocaine.

Why is prohibition at the heart of the argument?

Simple economics, say its opponents. It is simply a matter of supply and demand. If you squeeze the supply of a much-desired commodity – especially an addictive one – its price will rise sharply, and in an unregulated market, it can go sky-high. It then becomes too expensive for addicts for buy, and so they turn to crime or social deviancy on a large scale to feed their habits – burglary, shoplifting, prostitution. At the international scale, the profits are such that the trade is taken over by organised crime and whole countries are destabilised.

So just how big are the profits?

Transform's Danny Kushlick says: "In the cocaine and heroin trade, the profit margin is anything between 2,000 and 3,000 per cent, which enables organised criminals to turn what are effectively vegetables into commodities worth literally more than their weight in gold." A large number of prominent and entirely respectable economists have bought this argument, and insist that drugs prohibition is entirely counter-productive, just as alcohol prohibition was in the US in the 1920s - until it was eventually repealed.

They range from Milton Friedman, the US guru of the free market, to Adair Turner, former director-general of the Confederation of British Industry, current chairman of the Government's Climate Change Committee and forthcoming chairman of the Financial Services Authority. A lot of senior scientists are also strongly in favour of drugs legalisation.

Wouldn't the legalisation of cannabis or indeed any drug just lead people down the path to addiction?

That is certainly the position of its opponents; it is more or less the position of the Government and of the Tory opposition. Economists might be in favour; politicians are very wary of legalising drugs. There seems to be no widespread popular call for it. Its proponents say that although more people might become drug users, the harm done would be far less than the benefit gained by taking the world's Mafias and local criminals out of the equation.

So what are the chances that cannabis will cease to be internationally outlawed?

With the US running the show? Don't hold your breath.

Should cannabis be legalised on a world scale?

Yes...

* It would immediately take the supply of the drug out of the hands of violent criminal syndicates.

* Compared to alcohol and tobacco, which are freely available, Cannabis is not very harmful anyway.

* Any increased use of the drug would be greatly outweighed by the benefits gained.

No...

* It would be a first step to more widespread, and potentially disastrous, liberalisation of other drugs.

* It would lead to a great increase in use, which might put people on a "slippery slope" to harder drugs.

* Some forms of cannabis are very harmful and have been implicated as a cause of mental health problems.