Say the word “skunk” to teenagers and they may nod their heads, while politicians will shake their heads. Only a few brave ones will then whisper: “What exactly is skunk?” One public health study tried to ask teenagers about their skunk use but concluded that “it was unclear what people surveyed understood the term skunk to mean ... it is a confusing picture.”

To see that picture clearly through the fug, it is necessary to rewind the clock several decades.

In the 1970s there was a moral panic in America over teenagers smoking pot. At the time, the majority was imported Mexican Cannabis sativa plants, so, during the summer of 1975, blue-and-white American helicopters buzzed low over the Mexican marijuana fields, destroying the crops with toxic salt. At the time, President Ford thought that he had found a clever way to stop American teenagers from smoking “wacky baccy”. Moral panic over.

Yet that giant weedkilling operation didn't have quite the effect that the President was hoping for.

When the US Government sprayed the Mexican marijuana fields, imports dropped almost overnight. This, coupled with ever-increasing border controls, meant that dealers had to look to home-grown plants. But there was a problem: Cannabis sativa cannot withstand frost and won't flower reliably north of the 30th parallel. Furthermore, the plants are tall and hence difficult to conceal from the police.

The breakthrough for pot-smokers came when enterprising hippies returned from their travels with seeds from the variety of cannabis native to Afghanistan and India, Cannabis indica. Previously, few people had cared for the taste of Cannabis indica, but it was hardy and small. When Cannabis sativa was crossed with Cannabis indica, the industrial-scale home-grown market was born. And so, too, was skunk.

Steven Hager, a Sixties counterculture survivor and former editor-in-chief of High Times, a New York-based magazine that strongly advocates legalizing cannabis, says that the new hybrid cannabis was nicknamed skunk because of its unmistakably pungent smell: “The first seed company to breed indica into sativa was the Sacred Seed Company of northern California. Its most popular strain was called Skunk#1 - it is still one of the most circulated strains in the world.”

Since then, the production of hybrid cannabis has become a high-tech industry and, with estimated earnings of almost $50 billion (£36 billion) a year, easily America's biggest cash crop. This is what the British refer to as skunk. It has been the norm in America since the 1980s, although Americans refer to it by a variety of other names.

In the past, Britain's cannabis market was dominated by cannabis resin (“hash”) smuggled in mainly from the wild-growing cannabis sativa in Morocco. With the rising risk and cost of smuggling through ever-tighter border controls, though, by the late 1990s British criminals were copying the booming American industry: growing the “Cannabis sativa x indica” themselves.

Whatever worries people may have about skunk, air miles is not one of them - most of our cannabis is now grown here, mainly by Vietnamese gangs.

“Indoor cultivation has spread to the UK and other parts of Europe, which is why cannabis flowers are becoming more prevalent than hash in many places,” says Hager. “Indoor growing can be very profitable, since cannabis grows on trees and sells for the price of gold.”

In Britain this new type of cannabis - dry, mossy, green buds - was called skunk to distinguish it from the dark blocks of resin that came before. This is what the one in five 16 to 24-year-olds who smoke marijuana are almost certainly smoking.

Growers are now focused on increasing the strength of the “high”, which depends on the concentration of a chemical called tetrahydrocannabinol (THC). One way to do this is to select plants that are naturally more potent; another is to use lights to mimic the effect of autumn on the female plant. This causes it to produce more resin in a last-ditch attempt to pollinate itself before winter - and the resin is what makes it stronger.

David Crane, founder of the cannabis campaign organization The Hempire, describes this trend as “largely demand-led”. He adds: “Inexperienced kids think that strong stuff is the best stuff. They want to prove themselves. There is no one they trust to say ‘no, the gentler stuff is nicer'. And it takes a certain type of confidence to say to a dealer ‘This isn't really working'.”

Boys, it seems, are particularly keen on trying to outdo each other by coping with greater strengths of the drug - as they might with alcohol or curry. “Boys like to boast about the strength of their skunk,” says 18-year-old Katie.

Duncan, 27, a part-time drug dealer for two years, also characterises skunk use as a male-dominated pastime. “It suddenly became more available in 1999 or 2000. It was what everyone wanted,” he says. “The side-effects weren't really seen as a downside back then.

“For a lot of people it's about the strength. It was older brothers and mates' older brothers who introduced us - it's that way for everyone. I was 13. Smoking it is definitely a boy thing, I don't really know why. It goes hand-in-hand with computer games and sitting around.

“Skunk is definitely a young thing, too. I deal to people of my own age and everyone now is specifically asking not for skunk. They are people with kids and jobs who just want to have a smoke. Once you're out of uni and have to hold down a job, you get sick of it - and you need to be able to get out of bed in the morning.”

Another incentive for dealers is cost. “An ounce of hash or weed sells for about £40,” says Barry, a 28-year-old dealer. “An ounce of good-quality home-grown skunk will fetch anything between £180 and £200.”

The strength has certainly increased, but not as much as some media reports have suggested. In an analysis of drugs seized in Britain last year, Home Office scientists found that the old-fashioned Moroccan resin had a mean THC concentration of 6 per cent, while skunk was 16 per cent - rather like drinking a large glass of wine rather than a small glass of sherry. Its potency, they said, was not increasing year on year.

Some, like Steven Hager, argue that the stronger cannabis is, the healthier it is “because it means you'll smoke less and have fewer health issues due to inhaling smoke”. Whether teenagers regulate their intake in that way is unclear.

Finally, the scientists also found that British skunk had, compared with resin, very low levels of a chemical called cannabidiol. This has sedative properties, and experts such as Professor Robin Murray, a consultant psychiatrist at the Institute of Psychiatry and leading researcher into the effects of cannabis on mental health, have suggested that it could even work as an antipsychotic.

“We know that there is an increased risk of psychosis in people who use the old-fashioned type of cannabis,” he says, “but no study has yet taken into account the change in cannabis composition. Our clinical impression is that our patients choose to use the stronger varieties, in the same way that a typical alcoholic is not drinking shandy but prefers vodka or whisky. The average psychotic cannabis user is more likely to use skunk.”

One preliminary study compared cannabis users with just THC in their hair samples - typical of skunk use - and those with both CBD and THC, which showed that they were smoking old-fashioned cannabis. Those with just THC were more likely to show psychotic symptoms. Another preliminary study showed that CBD seemed to have some effect when given as an antipsychotic, and could even block the effects of THC.

“Probably CBD is not harmful and may actually ameliorate the effects of THC,” says Professor Murray. “The problem is that the general population's interest in this goes far beyond any funded research.”

It is not yet entirely clear what effect high does of TCH without the restraining effect of CBD will have on a generation of British teenagers. If this is the last unknown, it is the most worrying one.

What would you do if you found your child was smoking skunk?

Dr Marta DiForti

MD MRCPsych, psychiatrist at the Institute of Psychiatry

“If I found out my child was smoking skunk instead of cannabis, I would worry in the same way that I would if I discovered they were drinking whisky instead of beer. We know about the risks of cannabis - the effect on cognitive performance, learning and memory, and liability to psychotic experiences and becoming very suspicious and paranoid. It is likely that skunk has the same effects, but worse. But cannabis is not a monster. Like cigarettes, it is issues of frequency, duration and potency. It's a matter of public education.”

Martin Barnes

chief executive, Drugscope

“Parents or carers should inform themselves about the drug and try to keep the lines of communication open. While it's important not to overstate fears about skunk, all forms of cannabis are harmful and pose risks to physical and mental health. Its harm to mental health has been widely reported, and sometimes exaggerated, in the media. But there has been less attention drawn to other more common problems that cannabis use may cause for young people, such as the lethargic feeling. While cannabis is a harmful drug, it is important to recognise, without being complacent, that most users do not come to any significant harm."

David Potter CBiol MIBiol

CMIOSH, botanist and cannabis expert

“Skunk is no way as damaging as many of the other drugs out there, such as cocaine and the hallucinogenics, but I would be concerned, especially about a young person smoking it. There is the health risk but also the effect that it has on lifestyle, the apathy it seems to induce. It takes away their motivation, which will be a concern for a child's education. The message I get is that it helps you to relax and feel chilled, but young people have also said that they sit on their own smoking it and play their Nintendos.”

Dr Paul Broks

MSc, DPhil, CPsychol,neuropsychologist

“I would err on the side of caution. There is still not enough known about skunk but the evidence suggests that it can trigger psychotic symptoms in susceptible individuals. Before skunk came on the scene lots of people were cavalier about cannabis, me included. Now there are signs that the active ingredient, THC, may be intrinsically harmful, raising suspicions about milder forms of the drug. I'd be concerned if my children were heavy skunk users.”

Dr Tim Williams

MB ChB, MRCPsych, clinical lecturer in addiction psychiatry, University of Bristol

“Don't panic. If you look at the areas we use to measure addiction, such as control over use, desire, tolerance and withdrawal, skunk is not addictive. There is also no hard evidence that it is a ‘gateway' drug - so it doesn't mean they will move on to harder drugs. It's a peculiarity of the UK that cannabis is smoked with tobacco. The risks associated with that are well documented. I would ask the young person lots of questions about it - what they are using, how, in what environments, and what they get out of it. Drug users can block out all the negatives of what they do, and you can use the conversation to get them to see them.”