Dr. Conrad Murray's attorneys have for months suggested that Jackson could have swallowed propofol, which is normally given through an IV drop in hospital settings.
They told jurors in opening statements that they would present a theory that Jackson swallowed several pills of the sedative lorazepam and somehow self-administered the propofol - killing the singer before he had a chance to close his eyes.
On Wednesday, without jurors in the courtroom, attorney J. Michael Flanagan said he commissioned a study about the effects of propofol if swallowed, mainly to counter one by a key prosecution expert that it would have been negligible.
Flanagan said the defense study confirmed that the effect would be "trivial." "We are not going to assert at any time during this trial that Michael Jackson orally administered propofol," Flanagan said.
Deputy District Attorney David Walgren and Superior Court Judge Michael Pastor appeared surprised by the disclosure. It was unclear, however, if the defense planned to argue that Jackson might have injected himself with the fatal dose.
In recent days, Flanagan has focused his questions toward prosecution witnesses on the effect that lorazepam might have had on Jackson. Lorazepam was detected in Jackson's stomach contents after he died.
Murray has pleaded not guilty to involuntary manslaughter.
Prosecutors are in the final stages of their case against Murray, with three expert witnesses set to testify about their impressions of Murray's actions in the days and hours before Jackson's death and his efforts to revive him.
On Wednesday, Walgren called Dr. Alon Steinberg, a cardiologist who told jurors that Murray displayed gross negligence and repeatedly violated the standard of care.
He said Murray lacked the propofol monitoring or life-saving equipment when he was giving Jackson the anesthetic and other sedatives as a sleep aid. Steinberg said Murray should have never given Jackson the anesthetic outside a hospital setting, and criticized his attempts to resuscitate Jackson.
Steinberg said he based his testimony on statements Murray made to police about the dosages of propofol and other sedatives that he gave Jackson, and that the doctor had left the room for only two minutes before finding the singer unresponsive. He said based on Murray's characterization, Jackson could have been saved if the proper equipment were present and proper life-saving techniques were employed.
Asked by Flanagan whether his opinion would change if it was revealed that Murray had been out of the room for longer than two minutes, or had been on the phone while Jackson slept, Steinberg said no.
"He shouldn't have been on the phone," Steinberg said. "You can't talk on the phone. "
Authorities say Murray gave Jackson the fatal dose in June 2009. A medical examiner told jurors Tuesday that it was unreasonable to believe that Jackson gave himself the fatal dose of propofol when Murray left the room for only two minutes.
Dr. Christopher Rogers, who conducted the autopsy on Jackson, testified it was more likely that Murray overdosed the singer when he incorrectly estimated how much of the drug he was giving Jackson to induce sleep to fight insomnia. He said Murray had no precision dosing device available in the bedroom of Jackson's rented mansion.
"The circumstances, from my point of view, do not support self-administration of propofol," said Rogers, chief of forensic medicine in the Los Angeles County Coroner's Office.
Murray told police he gave Jackson only 25 milligrams of the drug, a very small dose that usually would have kept him asleep for no more than five minutes.
Flanagan spent more than two hours Tuesday trying to show on cross-examination that Jackson indeed could have self-administered drugs - not just propofol but the sedative lorazepam, which could be taken in pill form.
Flanagan suggested to the witness that once Murray had started an IV drip of propofol for Jackson and left the room, "it would be easy for someone to inject into that IV?"
"Yes," Rogers replied.
"But if they pushed it all at once, that can stop your heart, can't it?" the lawyer asked.
"Yes," said Rogers.
The implication was that if Jackson was desperate for sleep and in a hurry to administer more propofol before his doctor returned, he might have given himself a fatal dose.
But Rogers noted that investigators don't really know what happened when Murray left the room, so they were left to consider what is reasonable.
AP Special Correspondent Linda Deutsch contributed to this report.