LOS ANGELES (AP) -- An addiction expert testifying for the doctor charged in Michael Jackson's death told jurors Thursday he believes medical records showed the singer developed an addiction to a powerful pain medicine in the months before his death.

Dr. Robert Waldman told jurors that Jackson was receiving "above-average doses" of the painkiller Demerol in the months before his death.

"I believe there is evidence that he was dependent on Demerol, possibly," Waldman said. The witness said he also thinks Jackson had an addiction to opioids by May 2009, the month before his death.

Waldman said a symptom of Demerol withdrawal is insomnia. Jackson complained that he couldn't sleep as he prepared for a series of comeback concerts.

Attorneys for Dr. Conrad Murray have suggested Jackson was undergoing withdrawal from Demerol before his death. None of the drug was found in the singer's system when he died.

Defense attorneys contend Jackson gave himself a fatal dose of the anesthetic propofol, which they say he was taking as a sleep aid.

Authorities found propofol throughout Jackson's body during an autopsy, and they contend Murray gave the singer a fatal dose of the drug while using it to help him sleep.

Jackson received the Demerol shots from his longtime dermatologist, Dr. Arnold Klein, who has not been accused of wrongdoing and will not be called as a witness during the trial.

Waldman said he had not treated a case of Demerol addiction in recent memory.

Murray has pleaded not guilty to involuntary manslaughter in Jackson's June 2009 death.

The Houston-based cardiologist's attorneys plan to call a propofol expert later Thursday.

Murray's attorneys have yet to show evidence of how their self-administration theory would have been possible. Several prosecution experts have said the self-administration defense was improbable, and a key expert said he ruled it out completely, arguing the more likely scenario was that Murray gave Jackson a much higher dose than he has acknowledged.

The scientific testimony of Waldman and Dr. Paul White comes a day after jurors heard from five of Murray's one-time patients, who described the cardiologist as a caring physician who performed procedures for free and spent hours getting to know them. When Ruby Mosley described Murray's work at a clinic he founded in a poor neighborhood in Houston in memory of his father, tears welled up in the eyes of the normally stoic doctor-turned-defendant.

White and Waldman do not necessarily have to convince jurors that Jackson gave himself the fatal dose, but merely provide them with enough reasonable doubt about the prosecution's case against Murray.

Prosecutors have portrayed Murray, 58, as a reckless physician who repeatedly broke the rules by giving Jackson propofol as a sleep aid. But jurors heard a different description of the doctor Wednesday.

Several of the character witnesses called described Murray as the best doctor they had ever seen and highlighted his skills at repairing their hearts with stents and other procedures.

"I'm alive today because of that man," said Andrew Guest of Las Vegas, who looked at Murray. "That man sitting there is the best doctor I've ever seen."

Another former patient, Gerry Causey, stopped to shake Murray's hand in the courtroom and said the physician was his best friend.

A prosecutor noted none of them were treated for sleep issues, although Causey and others said they didn't believe the allegations against Murray.

Defense attorneys have told Superior Court Judge Michael Pastor they expect their case to conclude Thursday. Pastor has said if that happens, closing arguments would occur next week.

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AP Special Correspondent Linda Deutsch contributed to this report.

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