The night of Demi Moore's Jan. 16 911 call was even scarier than it sounded.

"Everyone thought she was going to die," a source tells the new Us Weekly of the emergency, in which Moore, 49, collapsed, and was "convulsing" after smoking an unknown substance.

It was the culmination of months of self-destructive partying (and drug use) for the newly single star, struggling to make sense of things as she nears 50 without husband Ashton Kutcher, from whom she split late last year.

"She's been really down, and she's surrounding herself with young people to make her feel better," an insider tells Us of Moore, who's been obsessed for years with staying young and thin.

"She's been mixing pills and Red Bulls, among other things," another source says.

One of Moore's favorite young stars to party with -- and flirt with -- is daughter Rumer Willis's longtime friend Zac Efron, 24, a hunk nine years younger than Kutcher, 33.

A wired, gaunt Moore "tracked down" the High School Musical star at a party in L.A.'s Venice neighborhood, an insider tells Us. "She seemed out of her mind at this party."

For additional details on Moore's spiral -- her calls and texts with Efron, how her three daughters are coping, her ongoing obsession with staying young, and what's next -- pick up the new Us Weekly, on stands Friday.