The 25-year-old singer was transported to a Los Angeles hospital where she is now being treated, and law enforcement says that she appears to have suffered from a heroin overdose, TMZ reports.
The representative didn't give details on what led to Lovato's hospitalization.
The pair's brother Kevin Jones tweeted: "Sending my prayers and thoughts to @ddlovato and her family right now".
This article discusses substance abuse and self harm.
She had tweeted earlier in the day that she was scheduled to appear on the Fox game show "Beat Shazam" on Tuesday night.
The singer, 25, has struggled with addiction, mental illness, and eating disorders for years, and has regularly come forward to discuss the stress of remaining sober.
Four months after celebrating six years of sobriety, Demi Lovato suffered a reported heroin overdose.
Meanwhile, Lovato's reps tell ET in a statement that she is "awake and with family". The singer had been sober for six years prior to her relapse, leading to the surprise release of "Sober", in which she apologizes for letting down herself, her family and her fans. The singer, who enjoys a huge fan base across the world has been receiving prayers on the social media from celebs and her well-wishers. "I wanna be a role model, but I'm only human".
Lovato's show in Atlantic City, N.J., set to take place on Thursday at Atlantic City Beach, was scrapped following her hospitalization.
She had just performed a concert in Paso Robles, Calif., on Sunday.
While some sources claim Demi is anxious about what her followers think, most of her fans have been incredibly supportive, and hope for the best for her. I am so happy you're alive. She went on to become a multi-platinum pop star, launching Top 10 hits like "Sorry Not Sorry", "Skyscraper", "Heart Attack".
Demi Lovato has been rushed to hospital over a suspected overdose.
"Obviously her health and well being are most important but we were really excited", said Andrea Grasso.
Singer Lily Allen called Lovato a "beautiful spirit" and expressed hope that Lovato "makes a full recovery soon". Another user wrote, "Please wake up baby, we need you and your music in our life". And may you receive the love so many have for you.
Like Joe, Nick Jonas also commented on the situation, saying, "Like all of you I am reeling at the news about Demi".
Tyra Banks added, "Addiction is not a choice".
A source tells ET that she is now "stable". The terms sat at the top of Twitter's list of trending topics in the hours following the news.