Birkhead said he was so concerned about her excessive use of medications that he once hid her Methadone only to be scolded by Howard K. Stern, who said she couldn't live without it.
"She would say, 'I'm in pain. I'm not a drug addict,"' he recalled.
When he suggested she was taking too many drugs, she replied, "I have a high tolerance because I'm in pain," he said.
Birkhead, a photographer and reporter, sat on the witness stand facing two people who had fought him for custody of Smith's child: the late model's lawyer-boyfriend, Stern, who is on trial in an alleged drug conspiracy, and Smith's mother, Virgie Arthur, who was in the audience.
Birkhead told jurors how he and Smith met in 2004 when he photgoraphed her and other celebrities at a Kentucky Derby party. He said he eventually moved into her Los Angeles home where a number of people were staying, including Stern, her personal assistant, and her son, Daniel.
Her neighbor was psychiatrist Dr. Khristine Eroshevich, he said, and sometimes Stern would call her when Smith wasn't feeling well.
Stern, Eroshevich and Dr. Sandeep Kapoor have pleaded not guilty to conspiring to unlawfully provide excessive drugs to Smith, prescribing to an addict and obtaining false prescriptions involving the use of fake names. They are not charged with causing her overdose death in 2007.
Under questioning by Deputy District Attorney Renee Rose, Birkhead said he noticed from the start that Smith was taking multiple medications including Methadone, Klonopin, Topamax, Ambien and Vicodin, among others.
He said Stern resented his presence in the house and tried to interfere with the relationship. He also said he saw Stern giving Smith medicine when she didn't feel well.
On the occasion when he hid a bottle of Methadone, Birkhead said he got a call from Stern asking where it was. "I was told by Howard she needed it to live, the Methadone," he said.
Birkhead was to continue his testimony Friday.
Outside the jury's presence, the judge said he would allow them to see snippets of Smith's appearance at the American Music Awards in 2005 and parts of a preceding interview in which she appeared drugged.
Superior Court Judge Robert Perry, however, refused to admit a controversial TV video clip of a bare-chested Kapoor nuzzling and kissing Smith's neck at a party following a gay parade.
Perry said the tape was clearly spliced.
"You are putting a man on trial and trying to destroy his career as a doctor," he told the prosecutor. "You had better well have good evidence and not be trying to do this with tampered evidence."
Earlier, Stern's sister took the stand and professed a lack of memory of shipments of prescription medications to him in the Bahamas.
Bonnie Stern, testifying under a grant of immunity from prosecution, identified e-mails between herself and Smith, who requested the shipments. Bonnie Stern said she couldn't remember having the items sent to Smith or her brother.
Stern's lawyer, Steve Sadow, asked about the personal relationship between Smith and Stern.
"She was his everything," Bonnie Stern said. "I only wish I would meet someone that loved me as much as he loved her."
E-mails admitted into evidence, however, revealed tensions between Smith and Stern in late 2006 just after Smith's son died and Smith gave birth to her daughter, Dannielynn.
"I hurt so much, and it seems Howard don't care," Smith wrote to Bonnie Stern. "I'm so angry all the time, so sad and mad what can I do. I don't want baby Lynn to see me so upset. I feel that Howard hates me. All he does is raise his voice and back talk me."
She said she felt a need to get away from the Bahamas but couldn't leave "because of our situation."
Bonnie Stern said there was a paternity suit involving Dannielynn at the time and a court case over ownership of the house where Smith was living in the Bahamas. Smith told her in e-mails to visit "your niece," indicating she believed the father was Stern.
After Smith died, a court found that Birkhead was the father.