Rosie Boycott: I know a joint is more dangerous now than in my day. But I still say legalise cannabis

When I was the editor of this paper at the end of the Nineties, I launched a campaign to decriminalise cannabis. I'd smoked it, most of my friends had smoked it, many politicians I knew well had smoked it and it hadn't done any of us any harm. Indeed, it seemed to me that cannabis was by the far the least harmful of all the drugs, legal and illegal, on offer today. At its worst it made you giggly, silly and unbearably hungry. And while it was not a good idea to drive when you'd been smoking, it didn't, unlike alcohol, make the imbiber violent and aggressive. But generations of young men and women were being turned into criminals because they'd been caught in possession of marijuana. Advocates of its continued prohibition spoke loftily of the "gateway effect": "Smoke cannabis today and tomorrow you'll be sticking needles in your arm." That was rubbish; the only gateway unlocked by cannabis was that the dealer who sold you the pot might then try to sell you something harder. Break the link and you break a vicious chain.

We launched the campaign in the autumn of 1997 and within days we were inundated with letters of support from policemen, teachers, probation officers and prison governors. Alastair Campbell said we were a bunch of old hippies searching for our lost youth, but we were confident that what we had begun was not only popular, but right.

But about a year ago, I started hearing stories about the new type of cannabis now widely available throughout Britain. Skunk is a genetically modified version of the hashish I used to smoke and for some teenagers, especially teenage boys, its effects are anything but benign. The first story I heard came from an old friend from the early Seventies. Her 20-year-old son, Daniel, had been diagnosed as suffering from psychosis.

Daniel was sent to Dr Mark Collins for in-patient psychiatric care. Dr Collins is a long-term expert on addiction and he is alarmed by what he is now seeing. "Cannabis psychosis mimics classic schizophrenia. Sufferers develop paranoia, delusional beliefs, hallucinations. What is very worrying is that these effects don't disappear when you stop smoking."

I saw that for my own eyes when I made a film about skunk for Tonight with Trevor MacDonald last summer and met Stephen. He was 26 years old, living at home in his parents' comfortable house in Kent. He'd started smoking heavily when he was 13 and by 18 he was hearing voices. "I hear them all the time," he said. I asked what they said to him. "That I'm no good, that I'm a failure. They never leave me."

So what has changed this hitherto relatively mild drug which so charmed my generation into the monster it has become today? The answer lies in the way it is grown. Old fashioned cannabis, the sort of stuff that I'd buy for a tenner in the late Sixties, and which came from Morocco or the Lebanon or Afghanistan, is now a rarity on our streets. In its place is skunk, grown at home from seeds bought over the internet: cheap, easy and very, very strong.

Behind a secondhand piano store a mile or so north of the Angel Tube station in London, there is a garden supply store with a difference. Struggle past the pianos and you're in a room containing things that look like Dr Who's Tardis. Lights of all sizes and shapes hang from the ceiling. On one wall, there are shelves containing bottles of brilliantly coloured liquids. Welcome to the skunk farm. For a few hundred quid, you can buy yourself an indoor, hydroponic unit, with lights that mimic day and night. Seeds with names like AK47 are available over the internet: fifteen cost £55. Just place them in the appropriate slots, add leaf-growing fertiliser for six weeks, change to the flower-growing variety, and, 12 weeks after you started, there is enough for your needs with plenty left to sell. The magic ingredient is THC - tetrahydracannabinol. The cannabis I used to smoke contained 1 to 3 per cent THC. AK47 has up to 30 times that amount. Rastafarians, well known as heavy dope smokers, won't touch skunk which, they say, is grown without love or light.

THC acts on the brain receptors and strong doses of it, used repeatedly over long periods, causes the brain to release floods of dopamine, which can unbalance a developing mind. Research shows that those who smoke skunk heavily have a four times higher risk of being diagnosed with a psychiatric disorder.

Of course not everyone will have a problem, but whereas in 1997 when we launched the campaign I could honestly say that cannabis had no history of serious harm, the same cannot be said today. Skunk and old fashioned hash, while coming from the same plant, are as different in their effect as brandy and beer. Because it so easy to produce, most of the cannabis on our streets has been grown hydroponically. And because there is a vast muddle about its legal/illegal status, the chance of any sensible information getting through to school children is remote.

While I was making the film for Trevor MacDonald, I spent time with a father whose son was suffering from skunk psychosis. As a professional film maker, he decided to make a short documentary about the potential effects of cannabis and took it to his son's school. The head refused to show it, saying that if he did, he would be condoning the smoking of an illegal drug.

Ignorance coupled with the plentiful and cheap supply of skunk is a recipe for disaster. Now, more than at any other time, the Government should seriously consider making cannabis completely legal. Ministers are naive in the extreme if they think that reclassification will do away with the problem. It won't. Unlike heroin or cocaine, skunk can be grown in every back room in the country. Turning it back into a Class 2 drug from its present Class 3 as the Home Secretary Charles Clarke apparently proposes would mean that the problems of skunk are pushed even more firmly underground, police time is once again compromised turning children into felons, and dealers only get richer. It won't stop people smoking.

A friend who is a child psychotherapist treats a lot of teenagers who have fallen foul of cannabis. She told me that she would rather her daughter took heroin as heroin addiction can be cured, whereas skunk psychosis often cannot. Unless we legalise cannabis and wake up to its very real dangers, we may unwittingly end up sacrificing more young minds in the name of law and order.