Whitney Houston back in rehab: Why is addiction so hard to beat?

(CBS) Rehab doesn't always work the first time around. Just ask Whitney Houston. The troubled diva recently entered an outpatient treatment program for a drug and alcohol problem - again.

"Whitney voluntarily entered the program to support her longstanding recovery process, Houston's representative told People magazine.

No word on where she's being treatment, or how long the treatment will last.

Houston, 47, first went into rehab in 2004. She went again in 2005. She declared herself clean in 2009, the Guardian reported.

Why do so many addicts have trouble getting clean - and staying clean?

"Addiction is a chronic disease," addiction specialist Dr. Omar Manejwala told CBS News. "Relapse is very common. Evidence suggests that the majority of the people need repeated treatment. If the initial treatment is only partially effective, we don't call that a failure."

Manejwala, the medical director of the Hazelden Foundation substance abuse treatment facility in Center City, Minn., indicated that the only real failure is failing to get treatment in the first place. And that's very, very common.

"Only about 10 percent of the people who need treatment for substance abuse get it," he said.

About 25 million Americans have a substance abuse problem, including about 19 million alcoholics.

The National Institute on Drug Abuse has more on the substance abuse and rehab.