The celebrity news website TMZ said police told them the 25-year-old was taken to hospital from her Hollywood home.
The American singer was reportedly treated with Narcan – an emergency treatment for narcotic overdoses.
Paramedics found her unconscious when they arrived at her home.
In a statement to Variety, a representative for the star said: “Demi is awake and with her family who want to express thanks to everyone for the love, prayers, and support.
“Some of the information being reported is incorrect and they respectfully ask for privacy and not speculation as her health and recovery is the most important thing right now.”
The singer, who has struggled with substance abuse for some years, released a song recently called Sober, in which she reveals she had relapsed after she celebrated six years of sobriety in March.
US chatshow host Ellen DeGeneres tweeted: “I love @DDLovato so much. It breaks my heart that she is going through this. She is a light in this world, and I am sending my love to her and her family.”
Ariana Grande posted: “I love u @ddlovato.”
The singer has been open about her addictions and her struggle with bipolar disorder and an eating disorder.
She has spent time in rehab and has admitted self-harming.
The Let It Go singer has worked with charities in the US to raise awareness of mental illness.
A spokesman for the Los Angeles fire department said they received a call at 11.22am for a “medical emergency” and took a 25-year-old woman from a property in Hollywood to hospital.