She opened up about it very candidly in her documentary Simply Complicated.
In the documentary, she admitted that went to rehab in 2010 for her addictions, but she relapsed two years later. She said: "I would sneak out and get drugs. I would fake my drug tests with other people's pee."
Just recently, celebrated six years of sobriety on Twitter.
However, just recently, a fan posted pics on Instagram which showed Demi holding a drink in her hand. While a big deal wouldn't have been made about the photo, it was the editing that made fans suspicious.
The photo was posted twice, by two of her close friends: Sirah and Matthew. In those photos, you couldn't see the drink in Demi's hand at all. However, when Hayley Kiyoko uploaded the exact same photo, the drink was visible in Demi's hand.
Below, the photo on the left was uploaded by Sirah and Matthew; on the right, by Hayley.
You can see that they're the exact same photo, with the same lighting/filter over the top. But in one, the drink is completely gone.
Not long after fans started calling Demi out for the photo, she replied to a comment on Instagram.
"I don't have to defend anything but it was red bull."