This is when your lungs get f**cked,' the cook splutters as he unscrews the top of a plastic bottle and carefully pours hydrochloric acid into the brown liquid. Gun tucked in his waistband, he reacts nervously to any sound, even the chickens rooting through the undergrowth. We have been told to run if any shooting starts but are not sure where to. At the bottom of the bowl, the acid and the brown liquid start to turn white. A minute in the microwave, and we have a kilo of cocaine.

We are in the depths of the Peruvian jungle watching coca leaves being converted into one of the most potent commodities on the planet......For 18 months I have trailed stories about the iconic drug of my lifetime. The journey took me into places I never believed you could get to: the deepest Andes in Peru to see cocaine being made, and the devastation of a culture; to the slums of Rio to see a city at war; and to the estates of Colombian drug barons to witness the unravelling of a state. We were seeking the voices of the men and the women behind its production and explore the effect on their lives of the West's 'war on drugs'. The degree to which my producers persuaded these 'criminals' to speak direct to camera is testimony to the outrageous confidence of some and the desperation of others.

In 20 years of filming around the world I have never taken cocaine, despite its ever-increasing availability. At the start of the project, I broadly supported the Blair government's more liberal policy of allowing people to have and to smoke cannabis. I agreed that hard drugs such as cocaine and heroin remained beyond the pale, accepting we had to fight them in every way possible. Though I have a number of friends who seem to be able to enjoy the occasional snort without problems, I have also witnessed the distressing effects of addiction.

This journey has revolutionised my views. I now believe that the tragedy we witnessed in Latin America has little to do with the damage the drugs do to people's heads. The tragedy is a result of the drugs being illegal. People will do a lot for a £34,000-per-kilo profit.

.....This journey has left me thinking the politically unthinkable. With an election looming, the Blair government has made the war on drugs a populist law-and-order priority, once again conflating the taking of drugs with the crime and violence that surrounds them. But it is the war itself that is the problem. The politicians rightly warn that demand will go up if it is legalised. Not good but not the nightmare they summon up. Neither cocaine or heroin is a cancer. In quantities it destroys your nose and is bad for your brain, but it very rarely kills - unlike that other addictive plant we can use legally: tobacco. Nor is it a direct cause of violence, like alcohol.

...Let's be honest. People try drugs, whether in the form of alcohol or pills, because they are fun. Tens of thousands of UK citizens regularly consume cocaine; hundreds of thousands more use other illegal drugs, completely discrediting the law. In his book Cocaine, Dominic Streatfield quotes the monetarist Milton Friedman: 'I do not think you can eradicate demand. The lesson we have failed to learn is that prohibition never works. It makes things worse not better.'

....over the past 15 years, the US has spent £150 billion trying to stop its people getting hold of drugs. In Britain and the US almost 20 per cent of the prison population is inside for drugs offences. So what is left? We can muddle on or we can legalise cocaine - and indeed all drugs.

....We should allow the farmers to grow coca and sell it for decent prices direct to government-controlled factories which can produce a high-quality product. And then it should be sold over the counter from registered chemists such as Boots to anyone over 18 at a reasonable, taxed price that does not encourage a black market. At least then we will know it is pure. Then we must attack demand by using some of the millions saved to invest in education drives that are honest. Look how effective a generation of anti-smoking education has been in bringing the public behind stringent restrictions on smoking in public, but not an outright ban.

...Yes, more people will try these drugs and there will be tragedies. But 30 years of the war on drugs have achieved almost nothing except to make a few people fantastically rich, to arm our inner cities, to criminalise a generation of users, and to leave tens of thousands of Latin Americans dead. As our cocaine maker in Peru happily told us: 'People want our cocaine because it is good and, for a while at least, makes them happy.'