“Rebel kids are crying out for discipline,” says Jack Osbourne, the son of the heavy metal behemoth Ozzy Osbourne and, for three seasons of MTV’s phenomenally successful reality show The Osbournes, a professional teenage tearaway. “When I was 6, I was a maniac. We had motorbikes at our house in the country and I was always pushing my luck: crashing, lying winded and in agony for a bit, then doing it again. I was never going to learn my lesson, ” says Osbourne, now 19. But boundaries can be hard to come by when your old man’s professional title is the Prince of Darkness.

Today Osbourne is nearly unrecognisable. Gone is the bespectacled Pugsley Addams lookalike who sulked around his family’s Beverly Hills mansion for our entertainment last summer. Transformed by rehab, Thai colonics and adrenalin sports, which are the subject of his new show on ITV2, the new Jack Osbourne is lean — down five stone from his 16st 7lb (105kg) peak weight — tanned and radiating health, with huge blue eyes that could belong to a Disney cow. Tell him he’s handsome, though, and he turns all bashful. “I’m not good-looking,” he protests, burying his face in his hands. “I’m rugged. I’m a mountain man.”

The youngest of three children of Ozzy and his band manager Sharon, Jack grew up at top private schools and the family’s Buckinghamshire estate, where he says: “My parents let me do what I wanted.” At the age of 10 he had a diagnosis of dyslexia and ADHD (attention deficit hyperactivity disorder), but he was still a relatively well-adjusted and integrated boy. All that changed when the family moved to Los Angeles in 1997. Enrolled, at the age of 11 in a special school for children with ADHD, Osbourne was issued the standard medicine, Ritalin. “I think ADHD is the most over-diagnosed disorder there is,” he says now. “Just because a child sometimes mouths off in class, it doesn’t mean that he has ADHD. Many things can be responsible, and they’re just dishing out speed as a solution!” Painfully self-conscious about his looks in a new city that worships beauty, Osbourne started drinking at the age of 12. Opportunity was not hard to find. Club owners would serve him “because of who Dad was”, and some of his father’s friends, too, would egg him on. “It’s a bit like getting a dog drunk,” says Osbourne, who is still legally too young to buy a drink there.

“People love seeing a kid drunk. It’s funny.”

A year later, he added marijuana to his repertoire. And, after giving up school and starting work in a record company at 14, he replaced Ritalin with the tranquillisers Valium and Xanax, and started taking the powerful painkillers Vicodin, Dilaudid and Lorcet — whose illegal sale has boomed in recent years; and they are easily available on LA’s party circuit.

“If you’re famous, you can get away with anything in that town. Go to a bar and say, ‘Anyone got a Vicodin?’ and 300 people open their bags and offer it to you. It’s like asking for a cigarette, pretty much.”

Next, the series The Osbournes put an overweight, frustrated and depressed 17-year-old Osbourne, plus family, in the faces of millions worldwide, while swelling the family coffers by a reported £18 million a series. Just what the doctor didn’t order.

“It was weird but, for some reason, when we were filming The Osbournes, at no point did I ever think, this is going to be on TV soon. It just didn’t register. Only when I had seen it, did I think, ‘Oh, I don’t really like that. Can we not do this any more?’ And then they pay you, and you go, ‘Oh, all right’.”

In the first weeks of the show, Osbourne was hit in the face by an unknown girl in a bar “for being a brat”. Even at the friendlier end of the scale, things “really started getting warped” when he went to the Emmy Awards. “Brad Pitt strolled up to tell me what he thought of the show. Not long afterwards, at a club, Britney Spears asked me to come over and say hello. Drew Barrymore came up to me. It was all totally insane.” Panicked and paranoid, Osbourne retreated deeper into his own private party.

Then, in 2003, Osbourne’s mother Sharon contracted colon cancer and Ozzy, terrified of losing her, broke nine years of hard-won sobriety. Osbourne Jr stepped up his self- medication, hanging out for days with his stoner cronies at his family’s Malibu beach- house. Until, looking around the room one day at the collection of thirtysomething heroin addicts who had become his crew, he took stock, went home and confessed the extent of his addictions to his parents.

As minors in the US can undergo rehab only in a hospital, that April Osbourne was admitted to the psychiatric wing of Las Encinas hospital in Pasadena, for rehab from alcohol, marijuana and OxyContin, a highly toxic, synthetic morphine-based painkiller which has been dubbed “hillbilly heroin”.

“It was a mental institution, pretty much a lockdown. You couldn’t go outside unless someone was with you; there are 20ft (6m) fences round the place. I was the oldest on my ward. There were kids of 14, 11. And I was the only one in there for drugs. Everyone else was in there for cutting themselves or hearing voices. The cutting kids, because they couldn’t find stuff to cut with, would scratch themselves until they made gaping wounds — blood everywhere.”

For seven days, Osbourne underwent a detox as the first stage of his treatment. “They whack you up with a bunch of Valium to make sure that you don’t go into a seizure and you sit there for seven days drooling. They detach your mind from the body so you don’t feel as much discomfort. And then — ping! They tell you ‘You’re fine now’. And you’re like, ‘Oh, really?’ ” Osbourne moved on to a clinic in Malibu where he spent 46 days of rehab in intensive individual and group therapy, beginning an Alcoholics Anonymous programme in which he chose Nature as his inspirational “higher power”. In sessions with his father, he deduced that he had “inherited Dad’s drinking, drugs and depressive genes”.

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