Mischa Barton's alleged drug and alcohol use has “alienated” close friends including Nicole Richie, it's claimed.

Reformed wild-child Nicole may have had her own substance abuse issues once upon a time, but even she has distanced herself from her former pal's self-destructive behaviour.

"In the last few years, her dark side has really come out," says a source of Mischa.

"Her friends, like Nicole, want to be supportive but they really can't be around her too much. She's too volatile."

"It makes us sad, but we're also like, 'Pull it together and get help already,'" the source adds to US People magazine.

Their lives have gone in separate directions now that Nicole is mum to 18-month-old Harlow and has another child with Joel Madden on the way.

"Nicole is focused on motherhood and her family," continues the source. "[She] considers [Mischa] a dear friend but she's also a mother who can't be around too much negative, unsafe behavior.

"Mischa is one of the reasons that Nicole is mostly with her other mom friends these days, not the party girls."

Insiders cite the infamous incident in 2007 when The O.C. actress was 'taken ill' at a Memorial Day barbeque as a turning point in her relationship with Nicole and other friends.

"Most of her Hollywood friends have distanced themselves from her [since that party]," says another source.

"After that happened, her friends thought it would be a wake-up call, but it wasn't."

Mischa is currently under involuntary pychiatric hold at Cedars-Sinai Medical Centre, after police were called to her Los Angeles home last Wednesday.

The actress' rep has denied reports her hospitalisation followed a three-day coke marathon.

Mischa at Paris Fashion Week in February