Demi Lovato will undergo an ''extensive program'' whilst in rehab.

The 25-year-old singer entered a rehabilitation facility last week, immediately after leaving Cedars-Sinai hospital where she had been treated for an apparent overdose, and sources have now said she is expected to stay in rehab for ''several months'' in order to go through a rigorous program which will ensure she gets as much help as possible.

A source said: ''She will be in rehab for several months and is doing an extensive program to ensure she gets the help she needs.''

The 'Sorry Not Sorry' singer temporarily left rehab a few days after entering in order to seek out further specialist treatment with a therapist in Chicago who specialises in addiction.

The trip was organised by the rehab centre, and sources say Demi has been doing ''much better'' in the short time she has been ''getting the help she needs.''

Speaking to E! News, the insider added: ''Demi is doing much better. She has become more open and is happy she is getting the help she needs, [as she now] grasps the severity of what happened and that she almost died.''

Meanwhile, it was previously reported Demi would be spending ''months'' - ''three, if not longer'' - in a remote rehabilitation facility.

An insider explained: ''She wanted a rehab [centre] that was unexpected and out of the blue, where no one knows where she is.

''Demi wanted to be in a drill sergeant-like situation where she could be challenged personally, mentally and physically without judgment.

''She wants to be submerged in an environment where her strengths are tested. She thinks this is the only way to break habit. She doesn't want to be catered to.''

This isn't the star's first trip to rehab as she previously spent time in a centre in 2010 to deal with similar addictions.

Demi had been sober for six years at the start of this year, but revealed in her new track 'Sober' - which was released in June - that she had relapsed.