The former Disney princess confessed she could barely go half an hour without snorting cocaine, and even smuggled the class-A drug onto planes.
During a candid interview – to coincide with the release of autobiography Staying Strong: 365 days a Year – the 21-year-old said her dependency was fuelled by her ability to manipulate people.
Demi explained: "I couldn’t go without 30 minutes without cocaine and I would bring it on airplanes.
"I would smuggle it basically and just wait until everyone in first class would go to sleep, and I would do it right there or I’d sneak to the bathroom and do it.
"That’s how difficult it got and that was even with somebody [with me], I had a sober companion, somebody watching me 24/7 and living with me, and I was able to hide it from them as well."
She said: "I was going to the airport and I had a Sprite bottle just filled with vodka and it was just nine in the morning.
"I was throwing up in the car and this was just to get on a plane to go back to LA to the sober living house that I was staying at.
"I had all the help in the world, but I didn’t want it. When I hit that moment I was like, it’s no longer fun when you’re doing it alone.
"I think at 19-years-old, I had a moment where I was like, 'Oh my God; that is alcoholic behaviour.' It’s no longer, 'I’m young and rebellious and out having fun.'
"It was, 'Wow, I’m one of those people. I gotta get my s*** together.'"
Opening up on her bulimia, Demi said the problems started when she was just "eight or nine".
She said: "I started overeating, compulsively overeating. I would bake cookies and then eat the whole pan. I went from doing that to being unhappy with my body.
"I went to just completely starving myself, and that turned into throwing up and starving myself. It was just this crazy battle going on inside of me.
"It got really difficult and I would throw up and it would just be blood, and it was something that I realised if I don’t stop this, I am going to die."
Demi eventually sought help in October 2010, going to rehab in a bid to turn her life around.
Dianna blames herself for many of the issues, explaining: "I had a terrible eating disorder that I had for years and I didn’t realise it and I had to face up to the fact that I was suffering as well.
"A lot of what Demi went through had to do with what she had seen growing up.
"I also had severe depression and I ended up asking for help, they did an intervention with me and said, 'Mum, you need to get help.'"
Demi has since soldiered on, teaming up with Simon Cowell for X Factor across the pond, as well as a recurring role on season five of Glee.
She also released her fourth self-titled album this year, earning a No3 spot on the Billboard.
It's not how hard you fall, but how quickly you get up...