The vibrant, life-loving woman they had known had been replaced with someone who was stressed out, tired and clearly not taking proper care of herself.
"Really, it was over the last year her friends saw a change," a source recently told PEOPLE as Moore endured the breakup of her marriage to Ashton Kutcher. "She wasn't sleeping as well, didn't seem to be eating and looked really gaunt."
As another source put it bluntly: "Demi is a mess."
TMZ reported that substance abuse was a factor in Moore's emergency hospitalization Monday night.
Moore, 49, who announced she checked into treatment for exhaustion, has not commented on the claims.
Sources close to both Moore and Kutcher tell PEOPLE that Moore's prescription drug use was a contributing factor to the end of her marriage to Kutcher, 33.
"It was a sticking point for Ashton," a source tells PEOPLE. "He wanted her to take care of herself and get a hold of things, and she wouldn't."
Adds the source, "Her family and friends have been really worried."
Rehab in the '80sAs a young rising star, Moore underwent rehab for drug and alcohol abuse in the mid-'80s, when director Joel Schumacher told her to get help before shooting St. Elmo's Fire.
After many years of sobriety, she still sticks to water and energy drinks at parties. But she likes to let loose – and some of her recent behavior troubled her and Kutcher's friends.
In August 2010, Moore dirty danced on stage at a Snoop Dogg show in Las Vegas. The impromptu stage show was an embarrassment to Kutcher, according to a friend.
Says a separate source: "[Ashton and Demi] both like to party, but she tends to wear it all over her. She can talk too much, get verbally abusive and weigh on your nerves."
Her thinner-than-ever appearance at recent events has also drawn concern, which Moore acknowledged as she discussed her "love-hate relationship with my body" in an interview in February's Harper's Bazaar.
"I think I sit today in a place of greater acceptance of my body," she said, "and that includes not just my weight but all of the things that come with your changing body as you age to now experiencing my body as extremely thin – thin in a way that I never imagined somebody would be saying to me, 'You're too thin, and you don't look good.' "
After Monday night, friends have never been more concerned. At least now, there may be a silver lining.
"Demi has had too much pride to come clean about her issues," says a source. "She wouldn't allow people to talk to her about them."
Now, it seems, she won't have a choice.
Reporting by ELIZABETH LEONARD, LINDA MARX and AILI NAHAS