When asked about their recent rapid weight loss, emaciated starlets trip out the same old excuses.

They're on a low-carb, high-protein detox or a juice fast; they're seeing this great new diet guru; they're working out; or they're really busy.

We've heard it all before. But in the past few months the smokescreen shielding the secret world of celebrity slimming has started to lift. It's being openly acknowledged that Hollywood is in the grip of a diet drug phenomenon.

"Trainers like me laugh ourselves silly listening to celebrities talk about their diet and exercise routines," says Jackie Warner, the co-owner of Sky Sport & Spa in Beverly Hills.

"If you want to get your body seriously skinny in three weeks, it's going to be difficult the natural way."

But there's nothing natural about Celeb-land's latest take on the trend to be uber-tiny that has seen some of the world's highest profile women shrink to less than six stone.

Poster girls for this scrawny silhouette include Victoria Beck-ham, photographed at the recent fashion shows looking painfully gaunt, while in the U.S., the pack is led by Nicole Richie, Lindsay Lohan, Mischa Barton and now Kate Bosworth, seen this week looking skeletal at a Dazed & Confused party in New York.

All five share the hallmarks of being seriously underweight ? chicken wing arms, deflated, sagging breasts and protruding hip and chest bones.

And three of them are "Zoe-bots", clients of the super-stylist Rachel Zoe.

Pin thin herself, she has championed the boho blonde, big sunglasses look in Los Angeles and masterminds the wardrobes of the Hollywood hot property she refers to as 'my girls'.

Many believe it's Zoe who is behind the emaciated look. But it's an accusation that she's quick to squash.

"I don't think it's fair to say I'm responsible because I'm a thin person, or that because I'm influencing their style I'm influencing what they eat," she says.

"There was this crazy rumour I was getting diet pills from Mexico and distributing them. I was like: 'OK, I've never even tried cocaine. I don't do drugs ? I'm too much of a control freak.' "

But despite her denials, fellow stylists believe the image Zoe is creating is the catalyst for a dangerous craze that sees many actresses resorting to illegal drugs in a bid to stay svelte.

"Now everyone in Hollywood thinks they look fat, every photo shoot turns into a therapy session," says one stylist.

Another adds: "If I get 20 samples and 15 are too small for the client, she needs a strong sense of self not to get caught up in it. Everywhere in LA there's someone slimmer, tanner, blonder." In essence, someone who has been given the Zoe treatment. Jackie Warner and fellow LA trainer Chad Mouton say more and more women are using the drug clenbuterol, a potentially lifethreatening steroid-like substance, to slim down ridiculously quickly.

Legal only for use in horses - it was developed as an equine asthma treatment - its use in professional sport is banned in Europe and the U.S.

But possession and purchase for private use isn't illegal and 'clen', as it's known, is frighteningly easy to buy over the internet. It was first used by bodybuilders who discovered it can burn fat while increasing muscle mass.

"It's a long-acting agent that increases the body's temperature and heart rate, which helps burn fat, even when the user is not exercising," says John McVeigh, an expert in substance use.

The reason behind its ability to build muscle, though, isn't fully understood. "It's not been tested on humans - this is an animal vaccine," says Harrison Pope, a Harvard psychiatrist who has been researching bodybuilding drugs since the Eighties.

"Most of the data available is veterinary, and it reveals a lot of bad reactions in rats. After taking the drug, their hearts started to stiffen. So it seems safe to predict that clenbuterol in high doses when taken by humans increases the risk of strokes and heart arrhythmia."

But despite the dizziness and palpitations, the ladies who don't lunch are getting it from their trainers, who sell it as a 'safe' alternative to steroids. "In looks-obsessed LA, the appeal of this drug to women is obvious," says Professor Charles Yesalis, head of sports science at Pensylvannia State University.

"It's short-sighted, but they see it as a quick fix to get the lean, toned physique popularised by actresses and models." Brooke Hailey, of LA's New Directions Eating Disorder Center, agrees. "I see more and more patients with bundles of disordered behaviours: overexercising, over-zealous exclusion of certain foods, the use of anti-anxiety medicines to curb the appetite - it's the culture and it's very, very accepted." Adderall is a drug that's finding its way into the cocktail mix of cocaine, smoking, dieting and purging that many actresses rely on to curb their hunger.

Introduced a decade ago as a prescription drug for the treatment of Attention Deficit Hyperactivity Disorder, its major side effect is weight loss because it suppresses the appetite for up to nine hours.

"For the jet- setting, hardpartying, over-extended girl, who naturally is concerned with her figure, it's become the miracle pill - one, however, with potentially dangerous consequences," reported W magazine recently. Even when taken as instructed, Adderall can cause psychotic episodes, depression and even serious heart problems.

"Last year, someone gave my cousin this doctor's number for weight loss," one woman told a reporter.

"He gave her loads of pills: a mixture of prescription and supplements. My brother had a party and you would not believe the state she got herself in - she soiled herself and was vomiting because of all those pills. The doctor never told her not to drink."

She was recounting this sorry tale in the changing room of her gym when someone overheard and asked: "Have you got his number?" Despite the fact that 20 deaths have been associated with the drug, women are willing to overlook the potential health risks in pursuit of a size double zero body.

"It isn't hard to get or illegal. Plus it's more chic to discreetly take an Aderall with a cocktail than to snort a line of coke," says one user.

Greta Angert, a Beverly Hills psychotherapist and eating disorder specialist, says Adderall doesn't carry a stigma because it is a prescribed drug. "But when someone is using it to lose weight, they should be treated for an eating disorder," she says.

In the U.S., with women only having to tell their doctor they are having trouble focusing to get a prescription for Aderall, and some physicians prescribing it to students to help them get through their exams, this drug is extending beyond Hollywood.

One New York pyschopharmacologist reported a trend in teenage girls selling their prescriptions to fellow students for e2 a pill. Victoria Beckham features prominently in the 'Thinspirational' galleries of proanorexia websites, where girls write in to coo about her 'beautiful protruding bones' and gaunt face, berating the fact they struggle to slim down to her size.

"Among my patients, Victoria Beckham is one of the top icons. As far as they can see, she gets invited everywhere, she's got plenty of money and a handsome husband," says Dr Dee Dawson of London's Rhodes Farm Clinic for Eating Disorders.

"It's not surprising that they associate her body shape with glamour and success." A report in the Journal of the Royal Society of Medicine said body image drugs were being bought by children as young as just 14.

"We've heard of steroid-like substances being sold at the school gates,' says Pat Lenehan, spokesperson for the Drugs and Sport Information Service.

The extent of the damaging effects on still developing bodies is not known. "If famous women are rumoured to take drugs, such as clenbuterol, ordinary people will be tempted to do the same," says Linda Wells, editor of beauty bible Allure magazine.

And at less than e30 for 30 tablets bought over the internet, these dangerous diet aids are barely more expensive than supplements and vitamins.

So where will it end, and who's really to blame? At the recent fashion shows, with activists calling for models to at least maintain a healthy BMI, the focus seems to be on designers.

But the lines between the runway and red carpet continue to blur as actresses become clothes horses for leading fashion houses.

At London Fashion week, the godfather of fashion Giorgio Armani went on record. "I never wanted to use girls who are too skinny. Unfortunately, though, the stylists and the media have interfered and the trend is for models who are incredibly thin," he says.

New York designer Michael Kors agrees that the culpability lies with the LA super stylists and says he is sending them more and more size zero samples. Recently, he's been getting requests for negative zero and even a negative zero size 2.

But while women are willing to overlook the potential health risks associated with popping a pill-sized slimming aid, new evidence is coming to light that this trend has a sting in the tail. While clenbuterol and Adderall create the desired sylph-like effect in the shortterm, long-term users are reporting sudden and uncontrollable weight gain.

Having messed with an user's natural metabolism, the drugs seem to lose their efficacy almost as quickly as they helped shift excess pounds. Jackie Warner has seen the body backlash first-hand. "A friend on clen lost weight, but after a while the drug stopped working," she says. "She was just eating tuna and told me: "If I eat anything, I just blow up.' "

It's an ironic twist that the threat of weight gain, rather than the risk of serious illness or even death, may hold more sway over young women's decision to dabble with Hollywood's most dangerous diet.