It’s still only April, but 2007 is proving to be a vintage year for the rehab industry. Lindsay Lohan, Britney Spears and Robbie Williams have given it the sort of Alist endorsement that money can’t buy, and barely a day goes by without some new celebrity “battling their demons”. As Britney checked out a few weeks back, Marc Jacobs, the American fashion designer was checking in.

For British celebs, a stint at the Priory (pictured below) in southwest London has become a showbiz rite of passage, a way to take time out from life in the spotlight. There are even rumours that certain PRs encourage their clients to enter rehab, so they can sell their story afterwards.

But rehab is no holiday. “It can be a bit of a shock,” says Rob Green, a former therapist at the Priory. “People will quite often turn up with a tennis racket expecting to be treated in a health spa.” The reality is quite different. “Clients”, as patients are euphemistically called, follow an intense daily schedule of workshops, lectures, one-to-one counselling and group therapy. There is no room service. In many cases, famous clients are expected to share bedrooms and help out with household tasks (referred to as “therapeutic duties”) as part of the recovery process. Television, iPods and sex are banned, and mobiles are confiscated on arrival.

Several years ago, I experienced the rigours of rehab first-hand when I went to a 12-step centre in Kent for heroin addiction. I knew that my luggage would be searched, so I arrived with a stash of downers sewn neatly into the hem of my dressing gown.

The first 24 hours were spent in the detox unit, separate from the main house. I refused all invitations to “come over and meet the others” and cowered sullenly under the duvet as the warmth of the heroin ebbed away. After a few hours, I had a visitor. The other inmates had heard there was a new arrival and, when I failed to appear, they sent Sam over to check me out. A stunning 6ft transsexual, she welcomed me like a grand society hostess as the nurses listened in. Then, drawing her mouth into a line like a bad ventriloquist, she muttered: “Got any gear, love?” The next day, I was thrown to the lions.

Rehab, I discovered, comes with its own complicated caste system. Your “drug of choice” defines who you are and which gang you hang out with. The alcoholics look down on the drug users and vice versa. The cocaine crowd think they are superior to the heroin users (more glamorous), while the heroin addicts scorn the coke-heads as lightweights. Near the bottom of the food chain is the confused housewife who has been abusing her prescription drugs, and even lower than that comes the unfortunate “foodie”.

“I remember how much we all hated the people with eating disorders,” says Fiona, who graduated from rehab and now works in films. “There was one bulimic we picked on mercilessly. We would say, ‘What’s the matter with you that you can’t keep a steamed pudding down?’ Eventually she was caught with her head down the toilet. She got kicked out for vomiting.” Anything legal (alcohol, prescription drugs, carbs) was considered uncool.

Just as there is competition in the boardroom, so there is rivalry among addicts: the lower you fall, the higher you rank in the pecking order. Everyone tries to win kudos by claiming to be the biggest, baddest junkie on the block: plummy trust-fund kids claim unlikely alliances with well-known south London crime families, and broken-down advertising execs claim to have snorted the entire GDP of Colombia. There is even one-upmanship about what strain of hepatitis C addicts are infected with: genotype 1 has the worst prognosis, so confers greater status of suffering.

Ultimately, rehab is like Big Brother: a group of damaged people from diverse backgrounds shut up in a house together. This creates a hothouse atmosphere in which friendships and hatreds blossom at unnatural speed and the air quivers with sexual tension. Anyone who imagines it’s like an upmarket dating agency should think again, however. One of the few people to find love at a clinic was Elizabeth Taylor, who met and subsequently married an alcoholic builder, Larry Fortensky. The inevitable divorce cost her a fortune.

REHAB: A SURVIVAL GUIDE

Thinking of checking in? Just remember the following:

Do get with the rehab lingo Sprinkle your speech with slogans, such as “a day at a time”, “stick with the winners”, “you’re only as sick as your secrets”.

Do perfect the long clinch that is the rehab hug Man-on-man hugs are usually accompanied by lots of vigorous shoulder patting to indicate heterosexuality.

Don’t exchange bodily fluids with other patients The stress that goes with sexual relationships can lead to relapse.

Don’t share your life story with all and sundry when you leave What is appropriate in group therapy is way too much information for your colleagues/nanny/manicurist.

Times recommends

• How I lost the gift of joy for a while
• How to get Coleen Rooney's post-baby body
• The A-Z of winter style rules