The US pop star, 28, says in a new documentary trailer: "My doctors said that I had five to 10 more minutes."
Lovato also told reporters she had been "left with brain damage and I still feel the effects of that".
Those effects include blurry vision that means she can't drive and made reading difficult, she revealed.
The former child star was taken to hospital in July 2018 after being found unconscious at her Los Angeles home. The overdose, reportedly of opioids laced with fentanyl, came a month after she revealed she had broken six years of sobriety.
In the trailer for her new YouTube documentary series Dancing With The Devil, she says: "I've had a lot of lives, like a cat. I'm on my ninth life."
Speaking to journalists on Wednesday, she revealed the lasting effects. "I don't drive a car because I have blind spots in my vision," she said.
"For a long time, I had a really hard time reading. It was a big deal when I was able to read a book, which was like two months later, because my vision was so blurry."
She said she was releasing the documentary "to help people who have been on the same path as I have", adding: "I wanted to set the record straight, and I wanted to reveal it all for my fans."