In the case of Smith, Kovner said she could have become addicted to Jello.
His testimony is significant because he handed his charts, notes and diagnosis to Dr. Sandeep Kapoor who bought his practice.
Kapoor, Dr. Khristine Eroshevich, a psychiatrist, and Howard K. Stern, who was Smith's boyfriend-lawyer, have pleaded not guilty to conspiring to provide Smith with excessive amounts of opiates and sedatives and to prescribing to an addict.
Yet Kovner testified he prescribed powerful drugs too and in large quantities.
"People deserve treatment," Kovner said. "There is an ethical and moral compulsion to relieve suffering."
Key in the government's case is proving that Smith was an addict and that the defendants knew it. But there would be no clear cut answer from Dr. Kovner.
"The legal definition and the medical definition are somewhat different," Kovner said.
In fact, the judge wanted to make the differences clear, so he printed out the legal definition and gave a copy to every juror.
Addict, it read, is a person who shows craving, plus impaired control over drug use, compulsive using or continued use despite harm. It does not include a person who seeks drugs due to inadequate control of pain.
Another issue for Kapoor's defense is the false name he used in prescribing drugs to the Playboy model as "Michelle Chase."
It was the name on the file he inherited from Kovner. Kovner said he got it from St. Joseph's hospital charts where he first treated her.
On Tuesday, Kovner testified that she complained of many physical pains with unknown sources.
He said he felt, "she was treating emotional pain with drugs designed for physical pain."
Kovner said that he warned Kapoor that Smith could be manipulative. Kovner also said he advised Kapoor not to socialize with celebrity patients.
Kapoor was photographed riding with Smith in a gay pride parade, and prosecutors said the social relationship clouded his judgment.
Kovner said that he told Kapoor in 2003 that Smith had chronic pain and an addiction to prescription drugs.
Superior Court Judge Robert Perry told the jury the law says a person whose drug-seeking behavior is due to a need to control pain is not an addict.
"Substance abuse is a lifelong disorder," Kovner testified, noting Smith told him she had been in rehab once for alcohol and Vicodin abuse.
Kovner described Smith as being very childlike, saying she acted like a teenager.
"Celebrities are known to often corrupt the judgment of good doctors," he said. "An outstanding example is Michael Jackson."
AP contributed to this report.