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 said he noticed from the start that Smith was taking multiple medications including Methadone, Klonopin, Topamax, Ambien and Vicodin, among others.
Birkhead, a photographer and reporter, sat on the witness stand facing two people who had fought him for custody of Smith's child: Stern and Smith's mother, Virgie Arthur, who was in the audience.
Birkhead told jurors how he and Smith met in 2004 when he photographed 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.
Earlier, Bonnie Stern, Howard K. Stern's sister, was also called on as a prosecution witness, but ended up telling the Los Angeles court of her brother's love for Smith and saying she didn't recall the shipments of drugs to him in the Bahamas.
Bonnie Stern testified under a grant of immunity from prosecution.
She identified e-mails between herself and Smith, who was requesting the shipments, but she said she couldn't remember having the items sent to Smith or her brother because she was overwhelmed by two businesses she was overseeing.
The subject then switched to the personal relationship between Howard K. Stern and Smith.
"Did you believe your brother was in love with Anna Nicole Smith?" Stern's lawyer asked Bonnie Stern.
"She was his everything," she said. "I only wish I would meet someone that loved me as much as he loved her."
Stern, Dr. Sandeep Kapoor and Dr. Khristine Eroshevich have pleaded not guilty to unlawfully provide excessive drugs to the Playboy model, 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.
The Associated Press contributed to this report.