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.