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 ¡ª 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 ¡ª 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.

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