Rosie Boycott: Skunk is dangerous. But I still believe in my campaign to decriminalise cannabis

I smoked my first joint in the summer of 1968. I was 17 and it was the summer of love: hot, sexy, the Rolling Stones performing for free in Hyde Park and the dope was plentiful and benign. It would come in from the Lebanon, Morocco or Afghanistan and I'd buy it in small lumps which looked and crumbled just like Oxo cubes.

Sitting on the grass in Hyde Park, armed with a packet of cigarette papers and the contents of a Benson & Hedges, I rolled my first joint. The dope made me happy. It seemed such a much better way to get high than my parents' nightly tipple of sherry or dry martinis.

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Everyone I knew in those days smoked pot and most people I know now have smoked at least once in their lives: some of them now run corporations and political parties, and there is no evidence that smoking pot ever hurt them. When I began a campaign to decriminalise cannabis at The Independent on Sunday in 1997 we were greeted with derision by the powers that be. Alastair Campbell memorably described us as a "bunch of old hippies still living in the Sixties".

But our campaign quickly attracted the attention of police officers, prison wardens and teachers who were by no means just a bunch of old hippies. Our points were simple: cannabis does less harm than alcohol; it does not lead people to violence, and no one smokes themselves to death (as they might drink themselves to death). Cannabis, not in itself an addictive drug, does not lead people to hard drugs but the criminalisation of it means that the person who sells you pot has a vested interest in leading you towards much more harmful and potentially addictive substances. Locking up young kids because they smoked dope meant we were making criminals of people who were, I believe, no more criminal than my sherry-tippling mother.

Our campaigning worked. In time the law was changed and cannabis was reclassified, making possession barely against the law. I am glad about this because I do not believe that we can ever contain the drug trade by making outlaws of the users and by allowing criminal gangs to control the supply.

But in one respect I have changed my mind. In 1997, I was confident that cannabis was an almost harmless drug. No drug, even caffeine, can be said to be entirely without its dangers. But I was talking about the pot that comes from the sun-filled fields of the Lebanon, Morocco and Afghanistan. Today's 30-times stronger variety - known as skunk - has been definitively linked with paranoid schizo-phrenia and psychosis, mostly among teenage boys who smoke heavily. It is now the most common form of the drug available on our streets because it can be grown so easily at home.

You can buy enhanced-strength cannabis seeds over the net. Simply type in AK-47 or Black Widow and you'll find yourself at a site which will instantly mail you enough seeds to start a small factory.

Last summer I visited a hydroponic supply store in north London located behind a piano shop. The piano area was musty and dimly lit, but once through a small door in the back, I was in something that was part garden centre, part pharmacy and part chemical repository.

Strange bits of furniture which outwardly resembled portable wardrobes opened up to reveal a complex system of lights and plastic tubes which carry fertilisers to the plants. By alternating light levels and a judicious use of chemicals, you can go from seed to plant in just eight weeks. The outlay is negligible. A single plant produces about an ounce of skunk, which costs between £100 and £120 on the street.

The dope I used to smoke that we campaigned to have legalised is now a rarity. Why bother with all the problems of importation if you can grow it in your bedroom as easily as I grew mustard and cress on blotting paper when I was a kid at school?

Psychologist Julie Lynn-Evans, who works with teenagers who have developed paranoia and schizophrenia from smoking dope, says that she would rather her children became addicted to heroin than skunk. At least you can completely recover from a heroin problem, whereas skunk can leave lasting damage. Teenage boys, whose brains mature later than those of girls, are particularly vulnerable.

Hearing voices is a familiar symptom. While researching a TV programme on the subject last year, I met a 20-year-old patient of Julie's and I asked him what the voices said. "Just real absolute junk... they don't want me to do that to them and I don't want them doing it to me but..."

He talked about them as though they were real. To him they were, holding conversations in his head which could go on for weeks, telling him he was no good, reinforcing messages of paranoia and low self-esteem.

Julie says that it is the most serious stuff on our streets today: "Once it has hit the frontal lobes of the developing adolescent, you just don't know whether they'll recover or not."

But how are people to know just what they are smoking? Teenagers are always going to smoke cannabis, just as they will always indulge in under-age drinking. But on today's chaotic streets, where cannabis doesn't come with a product-information label, it's like entering an off-licence and asking for a pint of alcohol without knowing whether you're buying beer or tequila.

The real dangers of skunk do not change my mind about legalisation. Indeed, I now think full legalisation to be more important, so that there can be sensible education about the possible dangers. We can never, ever hope to give out clear, straightforward educational messages about drugs while they remain illegal. We have no chance of ever controlling how drugs are sold and who they are sold to. Illegality drives the drug trade underground, exposing users to drugs - not just cannabis - of fluctuating strength and dubious origin, randomly dangerous in their inconsistencies.

Ending all prohibition on cannabis and all other drugs is not saying "yes" to drugs. Today's skunk is far cheaper and far more potent than what I smoked as a 20-year-old. And we are all paying an increasingly high price.

Unlike the old-fashioned cannabis of my youth, skunk makes people aggressive: they steal, break into cars and snatch phones. It makes everyone the victim but the true losers are our sons and daughters who literally, where skunk is concerned, risk losing their minds and themselves.