It's claimed Lovato was "uncooperative" when asked by paramedics about what drugs she had taken, The Blast claims.
The former Disney star was found unconscious inside her $8 million home in the Hollywood Hills just before 12pm local time on Tuesday (7am Wednesday NZ time), TMZ reported.
"LAPD and LAFD responded to a medical emergency at the 8000 block of Laurel View Dr. at 11:40am," a rep for the Los Angeles Police Department said.
They "transported a 25-year-old female patient to a local hospital".
Lovato was unconscious when paramedics arrived and was revived with Narcan, according to TMZ aand People, which is an "emergency medication to reverse the effects of a narcotic overdose". She is being treated at Cedars-Sinai Medical Centre in Los Angeles.
A source told People that Lovato had been battling personal problems. "Demi was never really clean and sober from all of her demons," the source said.
"She has been fighting depression and anxiety for quite some time — and is still in such a dark place," the source said.
"She was sober for awhile, but not completely sober for six years."
Lovato is "awake and responsive" according to her aunt Kerissa Webb Dunn, who runs her charity Demi Lovato Charity Boutique. "Thank you for your prayers and please continue praying for her full recovery both short and long term," Dunn wrote in a statement on Facebook, which has since been deleted.
Earlier, Dunn had urged her followers to "PLEASE PLEASE pray for my sweet niece DEMI LOVATO. She has been hospitalised and needs everyone's prayers desperately!!!! PLEASE PRAY she pulls through."
Lovato's medical emergency came hours after the star was photographed celebrating a friend's birthday in West Hollywood.
Inside Demi's personal struggles
The former Disney star has been open about her struggles with substance abuse issues. She previously spoke about living in a "sober" home after being released from rehab.
While she had been off cocaine for six years, last month Lovato released a song called Sober, where she admitted she'd relapsed.
Lyrics included: "I'm so sorry I'm not sober anymore" and "I promise I'll get help/It wasn't my intention/I'm sorry to myself."
Lovato previously completed a stint in rehab at age 18 for drugs, self-harm and an eating disorder before recovering in 2012.
In 2017 doco Simply Complicated, Lovato spoke about experimenting with cocaine at 17 and that she "loved it" right away.
"My dad was an addict and an alcoholic," Lovato said.
"Guess I always searched for what he found in drugs and alcohol because it fulfilled him and he chose that over a family."
She also revealed she nearly overdosed
"I went on a bender of like, two months where I was using daily," Lovato said.
"There was one night when I used a bunch of coke and I popped a few [prescription] Xanax bars, and I began to choke a little bit.
"My heart started racing, and I thought to myself, 'Oh my God, I might be overdosing right now.'"
Despite her rehab stints, Lovato said that she often lied about her sobriety in the media, and would frequently sneak drugs on planes in bathrooms, and conduct interviews discussing her sobriety while high.
Lovato, whose acting career has also included a recurring role on Glee, has in recent years developed a more mature image and has been politically active on issues including support for gay rights.
She was set to play a show in Atlantic City on Thursday as part of her Tell Me You Love Me tour, which has now been cancelled. She had performed at the California Mid-State Fair in Paso Robles on July 22.
