The 49-year-old actress has entered into treatment for "substance abuse and anorexia" at Cirque Lodge in Utah - which has previously treated stars including Lindsay Lohan, Mary-Kate Olsen and Eva Mendes - but has not yet had any interaction with other patients.
A source told RadarOnline.com: "She hasn't had any interaction with other patients yet. She is still in detox at the moment."
Charlie Sheen's ex-wife Brooke Mueller is also receiving treatment at the clinic and is said to be doing well as she tackles her drug addiction.
The source added: "Brooke is doing very well in rehab, this is the first time she has been in residential treatment, and it's been a very positive experience for her so far. Brooke will be at the facility for approximately 35-45 days in total.
"Brooke hasn't talked to or seen Demi. This is simply because they are in different stages in treatment."
Demi - who was rushed to hospital last month after suffering a seizure at a party - is expected to stay at Cirque Lodge for 30 days, while Brooke is due to leave the facility at the end of this month.
Last week, Demi's three daughters, Scout, 20, Tallulah, 16, and 23-year-old Rumer, as well as ex-husband Bruce Willis, were said to have encouraged the 'Striptease' star - who split from second spouse Ashton Kutcher in November - to seek professional help, but she reportedly refused to go to rehab.