Michael Jackson was probably addicted to a painkiller given by a dermatologist treating the star with botox in the months before his death, a medical expert has testified.

One of the symptoms of withdrawing from the painkiller, Demerol, was insomnia - the problem Jackson was being treated for when he died in 2009, the expert said at the manslaughter trial of the star's doctor, Conrad Murray, on Thursday.

"My opinion is that ... he was probably addicted to opioids," addiction specialist Robert Waldman said, adding that his assessment was based on medical records and other evidence presented about Jackson.

He was called to testify by Murray's lawyers, who presented medical records subpoenaed from the office of Arnold Klein, a Beverly Hills dermatologist who Jackson saw several times a week before his June 2009 death.

The records showed Jackson being treated with increasing doses of Demerol at sessions from April to June that year, to ease pain when he was being given Botox and other similar wrinkle-busting injections.

Murray's defence lawyers presented Waldman to the court in a bid to counter evidence implicating the doctor over Jackson's death from an overdose of the powerful sedative propofol.

The addiction specialist stressed the difference between drug addiction and dependency - addiction is a repetitive behaviour, while dependency is a physical need for a substance.

Cross-examined by prosecutor David Walgren, Waldman said that, based purely on the records from Klein's office, he would "probably not" diagnose Jackson with addiction, only dependency.

But he said the records combined with other evidence that has emerged during the trial, and public knowledge about Jackson, would lead him to believe that Jackson was probably an addict.

Murray's lawyers have tried during the trial - which could end within days - to suggest that Jackson was a desperate drug addict who could have killed himself by self-administering an overdose on the day of his death.

Prosecutors say he is guilty of involuntary manslaughter by giving Jackson a cocktail of sedatives, including propofol, to try to get him to sleep, and then abandoning the singer at the crucial moment, returning to find him lifeless.

Murray faces up to four years in jail if found guilty by the seven-man, five-woman jury.

-AAP