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You're no one in Hollywood until you come clean about your problems, which for Kelly Osbourne means opening up about her addiction to the painkiller Vicodin. In her new book, "Fierce," due out next month (and excerpted by the London Sun), Ozzy's little girl chronicles her descent into dependence and the four trips she took to rehab.

Kelly, 24, says she first experienced the "magical remedy" of Vicodin when it was prescribed to her after she had her tonsils out at 13, but it wasn't until 2002, when her mom, Sharon, was diagnosed with colon cancer, that her drug problem kicked into high gear.

"We all dealt with the news differently. [My brother] Jack went off the rails. He hardly visited Mum -- he's terrified of hospitals. He just went out and got high," she recalls. "My dad lost it. He drank and drank. He thought the love of his life was going to die."

Kelly followed their self-medicating lead.

"I thought I'd kill myself if Mum didn't make it," she says. "I was trying to be strong, so I took Vicodin to hide the terrible sadness."

By the time Sharon was on the road to recovery, "I was waking up and emptying six Vicodin into my hand," admits Osbourne. "Soon, I was taking 50 pills a day. Most people would overdose on 10."

Her parents knew something was up and tried to get her to take that first step by admitting she had a problem: "Mum would say, 'Darling, are you taking something? Please tell your mummy.' But I was good at lying."

Things hit the proverbial fan in late 2003 after Ozzy was seriously injured in a quad bike accident. Not long after the crash, Sharon confronted Kelly with incriminating photos showing her giving cash to her dealer.

"My mum shouted, 'You're a [bleeping] liar, Kelly,'" she recounts. "You've been lying to us and we're sick of your bull---t.'"

Osbourne says she "felt a sense of relief" with the outing. "I knew I needed help," she acknowledges. "They found 500 pills scattered around the room -- under my bed, in my handbag, in bottles."

A monthlong stay at celebrity-friendly Promises rehab in Malibu followed, but despite the "agony" she suffered during the detox process, she soon returned to her bad habits, leading to two more trips to treatment facilities, including a stint in same Los Angeles psych ward where Britney Spears would eventually spend time.

"I was so frightened -- it was full of crazy people," shudders Kelly. "As I curled up in a tight ball, the air was filled with ear-piercing screams. I didn't leave my room for five days. It's one of the scariest things I have ever done, but also one of the best."

But she continued to tumble off the wagon, bottoming out after returning to Los Angeles last fall after several years living in London.

"I wasn't prepared for the speed of my downward spiral," she confesses. "Within a month, I wasn't showering or brushing my teeth and my only relationship was with the pizza delivery man. The fact I didn't die is a miracle."

In January, Osbourne spent another 30 days in rehab, but this time, she apparently made a breakthrough.

"For the first time, I felt hopeful," says Kelly, who is now engaged to model Luke Worrell and is set to appear on the new season of "Dancing With the Stars." "If I think about relapsing again, I want to cry. I'm realistic. For the first time, I've learned my biggest lesson: There's a strong chance I will relapse. But because I recognize that, I feel like I've got the strength to fight it."

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Photos: The Osbournes