Birkhead took the witness stand Friday in a preliminary hearing to decide if three people should be tried for supplying the ex-model with drugs.
Birkhead moved in with Smith in 2004.
Birkhead says Smith's regular physician at the time was Dr. Sandeep Kapoor. one of the defendants. All three have pleaded not guilty.
Smith died in 2007 at a Florida hotel of an accidental overdose.