Birkhead took the stand in a preliminary hearing to determine whether two doctors and Smith's lawyer-boyfriend should stand trial on charges of conspiracy to illegally provide her with controlled substances.
Birkhead, who is from Louisville, Ky., told a Superior Court judge he saw bottles of Valium and other narcotics after moving in with Smith in 2004.
Birkhead testified that defendant Dr. Sandeep Kapoor was Smith's regular physician at the time and identified Kapoor seated across the courtroom.
The testimony was the first to name Kapoor as one of Smith's key doctors.
Prosecutor Rene Rose sought to show a video of Kapoor socializing with the former Playboy model at a gay pride parade and party. The video shows the doctor nuzzling and kissing Smith, Rose said.
However, Kapoor's attorney Ellyn Garafalo objected that the video was irrelevant to the case.
Garafalo previously told The Associated Press Kapoor is openly gay and never had a sexual or social relationship with Smith.
Superior Court Judge Robert J. Perry said he would review the video before deciding whether to allow it as evidence.
Kapoor, Dr. Khristine Eroshevich and Smith's lawyer-boyfriend Howard K. Stern have all pleaded not guilty. They are not charged with Smith's death in a Florida hotel in 2007, which was ruled an accidental overdose.
Birkhead fought Stern for custody of Smith's daughter, Dannielynn. Stern lost that battle when Birkhead proved by DNA evidence that the little girl was his.
In his testimony, Birkhead described a whirlwind romance that began when he met Smith at a charity event before the Kentucky Derby in 2003. Birkhead said he became Smith's personal photographer and moved into her home, sharing it with Stern.
Birkhead said he lived there until the fifth or sixth month of Smith's pregnancy.