Only fools and horses take the drug ketamine, if scriptwriters of The Archers are to be believed. Fools such as Jazzer McCreary, a Glaswegian transplanted to Ambridge and found unconscious on the toilet floor of the local pub, The Bull, prompting landlord Sid Perks to declare that the young tearaway had only himself to blame for his subsequent trip to intensive care. Like many a Radio 4 listener, no doubt, Sid can't understand why anybody would deliberately ingest a horse anaesthetic. Ketamine - also known as "Special K" - is, after all, a drug that vets use specifically to avoid kicks.

If you thought that Special K was just a breakfast cereal, then you are not alone. Ketamine was a relatively low-profile drug until its outing on Britain's longest-running soap opera. Supplying it to others is illegal under the Medicines Act. Possession, on the other hand, is not an offence because, unlike the better known ecstasy, ketamine is not yet classified under the Misuse of Drugs Act. Its misuse, however, is not confined to country boys. Frequenters of dance clubs, squat parties and rock festivals have been aware of it since the early 90s.

"A colleague of mine saw it in tablet form, cut with other substances, at last year's Reading festival," says Dr Simon Underhill, senior anaesthetist at Wrexham Maeler hospital. "He found people zonked out, in a very strange state."

But when they came round, would they suffer from permanent short-term memory loss, like Jazzer?

"Ketamine wouldn't do that on its own because it maintains heart and blood levels," says Underhill, one of several experts consulted over the Archers storyline. "My advice was that he should inhale vomit while under its effects. That could give him severe pneumonia which, in turn, would reduce oxygen levels and impair the nervous system."

This is not an unlikely scenario. Regular users quickly build tolerance and the more ketamine taken, the higher the chance of passing into a lengthy coma. "Some people do become dependent on ketamine in a way that resembles cocaine dependence, with craving," says Underhill. "However, there is no withdrawal syndrome if the drug is stopped."

With higher doses (200mg or more), unconsciousness may be preceded by numbness of the limbs, strange muscle movements and nausea. And the dangers are exacerbated if it is used at the same time as depressants, such as alcohol, barbiturates, heroin or tranquillisers.

One 26-year-old graphics professional from London, recently told Seethru.co.uk, a youth-orientated website, about the first time he snorted a short line of ketamine in the toilets of a club. "I found it hard to walk down the stairs," he says. "I had to hold on to the rail because I didn't feel in touch with my legs. It's hard to explain. Although everything seemed to be going chaotic, I didn't feel alarmed or paranoid. Just detached... it made me totally useless as I couldn't remember who I was with or talk to anyone."

Coming round from a ketamine-induced coma isn't much fun either. Users report extremely unpleasant hallucinations. "Those bad dreams have limited the drug's usefulness as a human anaesthetic in the developed world," says Underhill. "It puts the heart rate up and could cause strokes in those already suffering from high blood pressure. But if you are dealing with a 90-year-old woman bleeding from a gastric ulcer, it could be quite beneficial. It's also used occasionally as a painkiller in cancer cases where morphine has not been effective. And it can sedate patients in order to move them into the right position for a spinal anaesthetic. Apart from that, it's confined to emergency operations 'in the field'. Ketamine was used, for example, in amputations after the Moorgate tube disaster in 1975."

A decade earlier, US military doctors were using it in operations in the jungles of Vietnam. Like phencyclidine, better known as PCP or "angel dust", it was known as a dissociative anaesthetic. In other words, it could bring on what Harry Shapiro, of the charity Drugscope, calls "weird, out-of-body experiences". In other words, just what the West Coast hippies were looking for in the late 60s.

At higher doses, the drug is considered to have similar effects to LSD, including loss of control and delusions, such as imagining one could fly. The difference is that an acid trip can last for up to 24 hours, while the effects of ketamine wear off after an hour, leaving the user in a state of drowsiness or complete coma.

"It must be a bit like taking six drugs at once," says Shapiro. "A bit like LSD, a bit like heroin, a bit like amphetamines. But it is an anaesthetic. Eventually, all these things come out of the hospitals and research labs and on to the streets. Not that ketamine really emerged on this side of the Atlantic until the rave culture kicked in. At first people were taking it cut with ecstasy but only in the past three or four years has it become a drug of choice. It is hard to say why. In low doses, users report feeling euphoric and "rushy" for a while... But it doesn't strike me as a party drug because the euphoria can often be followed by numbness."

Part of the appeal, particularly for students and unemployed rock singers like Jazzer, is the low price - between £6 and £25 a gram. Conversely, there is snobbery attached, too. "There's a bit of a stigma about it being a dirty drug," says another user, a 36-year-old executive at a housing association in Greater Manchester. "Dope heads think it's unorganic and coke fiends think it's for losers."

There are obviously enough losers around for country vets to become very security-conscious. "We have to record and account for every milligram," says Anne Gibbs, who is a vet in Droitwich, in the heart of Archers country, and also an adviser to the programme. "If any went missing in suspicious circumstances, I would expect the police to come to us. I would say it's a very dangerous drug. Any anaesthetic carries a risk."

Even the North Yorkshire practice made famous by Alf Wight, better known as James Herriot, keeps ketamine under lock and key. "We don't carry it round routinely," says one of the present partners, Peter Wright. "We've used it for years on cats but only in the past 10 years has it been used for horses. In Alf's day we put them out with a chloroform mask, although that was a bit unpredictable. I've known a horse stagger out of a field and crash out in an adjoining rose garden."

It must have been more comfortable, and certainly more fragrant, than the toilet floor of The Bull.