The 56-year-old actress has recalled intimate details of her life for new memoir "Inside Out", with one section focusing on her relationship and subsequent marriage to Ashton. Writing about one time early on in their courtship when they went on vacation to Mexico, Demi said she was stunned when Ashton drank a glass of red wine and told her: "I don't know if alcoholism is a real thing - I think it's all about moderation."
The screen star, who had been sober for 20 years after struggling with her reliance on alcohol, then consumed a minibar-sized bottle of beer in a bid to prove to him she could indulge without going overboard.
"I didn't think, 'This is a kid in his twenties who has no idea what he's talking about'," she wrote in. "Instead, I cast about for justifications for his argument."
Demi went on to say the incident of breaking her sobriety led to her turning to alcohol when she suffered a miscarriage at six months pregnant. She quickly became reliant on substances such as alcohol and some drugs, but Ashton apparently began to grow tired of her drinking, with things coming to a head when she was celebrating her birthday in Puerto Vallarta, Mexico at "Girls Gone Wild" creator Joe Francis' home. An overindulgence in alcohol at the time led to Demi passing out in the hot tub, with Ashton allegedly getting angry with her because it could have led to fatal consequences. Calling his angry reaction "'confusing", Demi added she didn't know why he was so cross as he "had encouraged (her) to go in this direction" when he first made the claims about alcoholism during their Mexican vacation.
Ashton has yet to publicly respond to the claims made by Demi in "Inside Out", due for release on 24 September.
He and Demi were married from 2005 to 2013. He has since married his "That '70s Show" co-star Mila Kunis, with whom he shares two children.