Lisa Tseng is charged with three counts of second degree murder and 21 other felony counts that could send her to prison for life if convicted. The hearing will determine if there is enough evidence to go to trial.
Tseng is accused in the 2009 deaths of patients Vu Nguyen, 29, of Lake Forest, Steven Ogle, 25, of Palm Desert and Joseph Rovero III, 21, an Arizona State University student from San Ramon.
Prosecutors said Tseng overprescribed drugs like Xanax, Oxycontin, Vicodin and Adderall to make money.
Witnesses took the stand Monday, including Tseng's office assistant, Gloria Rodriguez, who described procedures at the office and the crowded lobby. She said that on an average business day in 2009, there were "probably between 30 and 40 people" in the waiting area.
The Drug Enforcement Administration alleges Tseng wrote more than 27,000 prescriptions over a three-year period at a rate of 25 per day with minimal questions and only cursory patient examinations.
Prosecutors have more than 80 witnesses who could be called to testify, including former patients, family members, medical experts and law enforcement.
The osteopath has pleaded not guilty to the charges. Tseng came under scrutiny in 2008 when a pharmacy reported problems with Tseng's prescriptions.
Tseng has settled five wrongful death lawsuits dating back to 2007 and turned over her medical license.
"Dr. Tseng cares deeply for her patients and looks forward to her opportunity to vindicate herself in court," her attorney said.
The hearing is expected to last 12 days.