From PEOPLE Magazine Click to enlarge
THIS season she landed the role of determined drug counselor Ellen Marks, who's fighting to keep Beverly Hills, 90210's Dylan McKay from self-destructing on cocaine. Let's say she knows the material: In 1980, 20-year-old Mackenzie Phillips's own raging cocaine habit got her fired from a seven-year gig on the CBS sitcom One Day at a Time. The following year she and her dad, John Phillips, of the '60s group the Mamas and the Papas, did a father-daughter rehab stint at a New Jersey psychiatric hospital, disclosing that their family drug habits had cost them more than $1 million over the years.

After detox, Mack began preaching against the ravages of drugs. But despite her public stance, she now admits that rehab didn't stick. "I was lecturing on not doing drugs, and after the lecture we'd drink wine with dinner," she says. "It took me 2½ years before I started doing coke again." Virtually blacklisted as an actress, she spent the next nine years as a rocker, touring the world with a reconstituted Mamas and Papas. She and the group's guitarist, Shane Fontayne, have been housemates for the last 13 years and have a 7-year-old son, also named Shane.

In retrospect, a rocky road might have been foreseen for the Hollywood wild child who made her film debut at 14 in American Graffiti. "Mack got jaded pretty early," recalls stepmother Michelle Phillips, whose six-year marriage to John began when Mackenzie was a 4-year-old. "She had men in their 40s in platform shoes with glitter on their eyebrows taking her to the Whisky a Go Go when she was just a teenager."

But in 1992, tired of the fast life, Mack, now 35, quit the tour, retired to her and Fontayne's contemporary home in Stroudsburg, Pa., and began what she says is her most serious attempt to give up drugs for good: "All I could see was more humiliation ahead—and getting worse." Young Shane is well-versed in his mother's addiction and attends AA meetings with her. Says Mackenzie: "Shane is adamant that he'll never take drugs." Now in L.A. trying to resurrect her career, Mackenzie, who frequently visits back home, gets moral support from half sister Chynna Phillips and recently reconnected with One Day's Valerie Bertinelli. "There's been so much tragedy in Mack's life," says Mama Michelle. "I want to see her happy, but it's got to happen this time. I don't think she'll get another chance."