"My opinion is that .. he was probably addicted to opioids," said Waldman, noting that withdrawal symptoms from Demerol include insomnia -- from which Jackson was suffering when he died from an overdose of powerful sedatives.
Waldman's testimony was punctuated by repeatedly testy exchanges in court with prosecutors, who last week closed their case after four weeks of witnesses heavily implicating Murray over Jackson's death.
The addiction expert was called to testify by Murray's lawyer Ed Chernoff, who presented medical records subpoenaed from the office of Klein -- who has been mentioned before in the five-week trial, but not called as a witness.
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.
The addiction specialist stressed the difference between drug addiction and dependency -- addiction is a repetitive behavior, while dependency is a physical need for a substance.
"I believe there is evidence that he was dependent upon Demerol," he told the court.
Cross-examined repeatedly 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 that 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 to suggest that Jackson was a desperate drug addict who could have killed himself by self-administering an overdose of the clinical anesthetic propofol on the day of his death.
Prosecutors say Murray committed 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.
The defense began presenting its witnesses Monday, when a doctor recounted how Jackson feared he might not be able to perform a series of 50 comeback shows in London, and begged for something stronger than sleeping pills.
On Tuesday a nurse who treated the singer told how he begged to be given a propofol drip two months before he died, saying medics had assured him it was safe.
On Wednesday a string of former patients of Murray praised his "caring" attitude and denied he was greedy for the $150,000-a-month salary he was being paid by Jackson.
Murray faces up to four years in jail if found guilty by the seven-man, five-woman jury.