Dr Robert Waldman was speaking at the ongoing Conrad Murray manslaughter trial, as a witness called by the doctor's defence team.
Murray's lawyers are alleging that Jackson was taking the drug Demerol from a Beverly Hills dermatologist, which clashed with Murray's attempts to help the singer with his sleeping problems, and eventually led to his death.
Medical records state that Jackson had taken 900 milligrams of Demerol in three days during May 2009, from Dr Arnold Klein.
Klein also gave Jackson Botox and Restylane treatment for wrinkles and excess perspiration over several months in 2009.
Waldman told the court that there was a substantial "possibility" that Jackson was addicted to Demerol.
"I believe there is evidence that he was dependent on Demerol," he said.
However, in cross-examination, Waldman couldn't fully say that Jackson was dependant on the drug, and that he was also not an officially recognised addiction specialist.
Murray was moved to tears in court yesterday when several past patients were called as character witnesses to defend his case.
The trial will soon come to a close, with Murray facing up to four years in prison if found guilty.
> Michael Jackson 'begged for propofol drip', hears Conrad Murray jurors