It's not as if he doesn't have anything to live for: He could become one of Hollywood's biggest and most enduring names. It's not as if he is in denial about his addiction; instead, he seems to recognize the severity of the problem. It's "like I've got a shotgun in my mouth, with my finger on the trigger, and I like the taste of the gun metal," he explained to a judge last year. He seems to know that his problem is screwing up his life, and has repeatedly announced his desire to quit drugs. He has attended plenty of treatment programs. And he has the financial and professional resources to get medical help.

In recent months, after his well-publicized struggles and jail time, his career righted itself. He scored a recurring role on the popular show "Ally McBeal." He was signed to star in a new movie, "America's Sweethearts," with Julia Roberts, Catherine Zeta-Jones and Billy Crystal. Mel Gibson was slated to direct him as the lead in "Hamlet" next year in a Los Angeles stage production.

So why did he again find himself under arrest Nov. 25, charged with possession of cocaine and methamphetamine, and why can't he kick the habit?

In part, because it's not a habit -- not something that can be overcome by sheer willpower, according to Jeffrey Hon of the National Council on Alcoholism and Drug Dependence. "Drug addiction is a chronic medical condition -- it's a behavioral illness just as much as diabetes or asthma is," he says. "It's not as if an addict can go into treatment and suddenly get cured once and for all." George Koob, a neuropharmacology professor at Scripps Research Institute, concurs: "Whether you're a movie star or a truck driver, you've got to understand that you always have to work at staying clean and constantly monitor yourself, just like a diabetic has to," he says.

In fact, Downey's superstar status has probably detracted from the treatment he has received, says Joseph Pursch, a psychiatrist who specializes in addictions and has treated high-profile people such as Betty Ford and astronaut Buzz Aldrin. That's because doctors and caregivers -- who want their famous patients to like them and recommend them to other big-deal clients -- often cut corners for VIPs, like letting them out of treatment before they are fully recovered. "I call it the 'VIP syndrome,'" he says.

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