People who become actors and musicians tend to be extrinsically motivated, and their sense of self is bound up in what others think of them. It results in emotional highs and lows, and often depression. "If you are needy in that sense, you can become obsessed with your press cuttings, obsessed with what the critics and fans think of you," Ross adds. "As soon as the critics start turning on you, which happens to all artists at some point, you're on an emotional roller coaster."

Another factor is the environment celebrities find themselves in. "I played in a band for years and we didn't have any money, but we always had drink," Ross says. 

"Everywhere we went there would be a free bar. The drink was the first thing that was laid on. Maybe you wanted a cup of tea, but that wasn't really an option."

Stars are also expected to act in a certain way, leading the charge towards the free bar and partying hard. On the road, they are often cut off from the stabilizing effect of friends and family. 

"People who live in poverty, specially in inner cities, can get in a mess with drugs. Celebrities suffer a poverty of meaningful relationships," Ross says. 

'That's why they sometimes leave the house looking ridiculous' 

"People you have known for a long time will tell you that you look stupid. Nobody would ever tell most of the celebrities that they look stupid. That's why they sometimes leave the house looking ridiculous, because most people will always tell them that they're great."

Celebrities can often be surrounded by people who collude in their addiction, says Dr Michael Scott, medical director of Sierra Tucson, a treatment center in Arizona. The clinic, which specializes in addiction, is known for its celebrity clients, who have included Ringo Starr, Rob Lowe and Michael Douglas.

"Celebrities are, directly or indirectly, supporting a number of people, such as their agent and everybody else they work with," Scott says. 

"These people have a real vested interest in how well the star does, and they tend to keep things quiet when they shouldn't. It's out and out enabling. If the celebrity gets help it could compromise their job and they don't want that to happen. They tend to be one of the major obstacles."

Addiction for "civilians", as Liz Hurley charmingly referred to non-celebrities, still carries an enormous stigma. People struggle to hide their drinking and drug habits out of fear that they may lose their jobs. 

With celebrities, not only is addiction widely tolerated, but admitting to a destructive habit can jump-start a failing career in terms of sympathy and press coverage. It's a subject that can guarantee a place on a chat show sofa and a huge book deal.

"I'm sure the marketing people will tell you that it sells as much as any other press," Ross says. 

"The evidence of that is the bandwagon-jumping. As soon as somebody goes into rehab, suddenly you see a dozen people coming out with their own rehab stories."

The psychologist doubts that all the stars telling such tales have actually had a medical diagnosis. 

"I do think that there is a social and cultural threshold now, which is quite low, whereby you can just say you're addicted to something for functional reasons, usually to get yourself off the hook if you've done something bad.

"If celebrities do something that is socially unacceptable - for example, with respect to sexual behavior - they will often claim to be addicted. Michael Douglas seemed to be just another guy who cheated on his partner. But he had access to good lawyers and good PR, and the explanation that he was addicted emerged and people felt sorry for him.

"I think Kate Moss had to originally come out and say she had a problem with cocaine for PR reasons. I wouldn't know whether there was a clinical problem, or whether it was just recreational drug use and the main problem was that she got caught," Ross says.

"It's very hard for a public figure to get better when they are in the public eye," Scott says. "In group therapy sessions they are afraid that it will leak, which it often does, or they are embarrassed to say it in front of their fans.

"In order to get better these people often have to bare their souls, and it takes a very trusting environment to be able to do that. That's why we are fortunate, because we are able to maintain confidentiality pretty well. We put a lot of faith in the patients. We address the importance of confidentiality all the time and stress that their personal recovery depends on the confidentiality of everyone there."

Celebrities are treated like everyone else and have to attend group therapy sessions at the clinic. "They have to be treated the same because we are treating the person and no matter what job they are in," Scott says. "Part of recovery is to address who they are and it's often hidden in layer on layer of external garb." - The Independent