Demi Lovato's bulimia was sparked by school bullies who branded her "fat".
The 20-year-old former Disney star - who was admitted to rehab in 2010 for help with an eating disorder, self-harm and depression - believes her problems started at the tender age of 12.
Talking about her tormentors, she said: "They called me a whore and told me I was fat and ugly. I shouldn't have listened, but I took it to heart and it hurt. I thought maybe I didn't have friends because I was too fat."
Deciding to take action, she began making herself throw up and lost more than 28lbs in six months, taking her weight to just 88lbs.
She said: "I'd make myself sick up to six times a day. My mum was worried, but because I was going through puberty I was having growth spurts so she assumed that was why I was thinner."
But when losing weight didn't make her happy, she turned to self-harm as a way to deal with her pain.
She told Fabulous magazine: "It started with my wrists. People saw that, so I cut in places they couldn't see.
"You do it because you feel so bad inside. You don't know how to take it out other than on yourself."
Her demons eventually got the better of her and she ended up punching a backing dancer on a flight during the Jonas Brothers' tour, which was followed by her prompt admission to rehab.
She said of her treatment: "It wasn't my idea, but I didn't fight it.
"It was really, really hard and scary. I was homesick and lonely and several times I thought f**k it, I'm leaving. But my mum told me I would regret it. This was my only chance.
"I had 14 hours of therapy a day. I listened to music and learned to knit. When I finally left, it was like being let out of prison."