Birkhead is helping lawyers for the State of California in their case against the former Playboy beauty’s doctors, who are accused of illegally supplying her with prescription pills before her death from an accidental overdose in February 2007.
According to legal documents filed by prosecutors, Birkhead has “personal knowledge of Anna Nicole Smith’s drug activities, gleaned from years of personal observation”.
They add that Birkhead is convinced Smith was an “addict” because he “personally observed her using controlled substances in an uncontrolled way over a long period of time”.
But prosecutors are facing a challenge from attorneys defending the doctors, who claim Birkhead isn’t “qualified” to determine whether Smith met the legal criteria of a drug addict, reports TMZ.com.
Birkhead is the father of Smith’s three-year-old daughter, Dannielynn.