Doctor Conrad Murray, late Michael Jackson's personal physician, is pictured during his arraignment on a charge of involuntary manslaughter in the pop star's death, in Los Angeles, California, in this January 25, 2011 file photo.[Photo/Agencies]

Michael Jackson was physically exhausted from a day of grueling rehearsals for his marathon 50-night comeback tour. But his nightly battle with insomnia had just begun. After showering and getting into bed, he called for his "milk," a powerful drug he had been using to escape into unconsciousness.

Jackson saw the anesthetic known as propofol as his salvation. On June 25, 2009, it became the King of Pop's death potion.

How he overdosed in his mansion on a drug intended for hospital use is at the center of the manslaughter trial this week of the doctor he hired to be his highly paid personal physician for the "This is It" tour.

Testimony about the drug is expected to dominate the trial of Dr. Conrad Murray, a Houston cardiologist who has pleaded not guilty to a charge of involuntary manslaughter in Los Angeles Superior Court.

The prosecution claims Murray was grossly negligent in giving Jackson propofol at home without proper lifesaving equipment available and then left the room long enough to find his patient not breathing when he returned.

His defense team claims the singer, desperate for sleep, swallowed an additional dose of the drug when his doctor was out of the room.

Getting to the truth of it will come down to sometimes technical testimony from an array of medical experts, pathologists and even the police officers and paramedics who inspected Murray's equipment in the bedroom where Jackson went into cardiac arrest.

The defense theory, based on evidence that a trace amount of propofol — .13 milligrams — was found in Jackson's stomach, may be a hard sell.

The drug is administered intravenously, usually during surgery. Scientific witnesses may be asked to explain how it could have gotten into his stomach. Some doctors say ingesting it orally is almost unheard of.

"It's an odd, on-the-edge defense theory," said Dr. Gil Tepper, chief of staff at Miracle Mile Medical Center in Los Angeles. "It would not put you to sleep and it would move through the system very rapidly, causing awful diarrhea."

There are few authoritative studies and few statistics on deaths caused by the drug. It remains uncertain if the judge will allow a Chilean doctor's study of students who voluntarily drank the drug or a study of pigs who received it rectally.

The prosecution has a key expert witness, as well as forensic experts from the Los Angeles County coroner's office who are considered at the top of the field.

The defense boasts an advantage in one of its lawyers, J. Michael Flanagan, who says he is the only California attorney ever to try a propofol death case.

Flanagan represented one of a pair of nurses charged with killing a cancer patient who died after propofol was allegedly given without proper authorization by an anesthesiologist. Flanagan's client was acquitted and the other nurse pleaded no contest to a lesser charge.

Abuse of the drug is said to occur among medical professionals but rarely among patients.

In anesthesia, propofol is known as something of a wonder drug. Tepper said it's ideal for short term procedures such as colonoscopies or cataract surgery. Patients report feeling energized when they come out of it and there is no grogginess.

"But it's absolutely not recommended as a sleep aid," said Tepper. "You would never reach restful sleep and you would have to have constant monitoring."

In a hospital setting, Tepper said the doctor would have heart and blood oxygenation monitors as well as surgical equipment to do intubation if the patient stops breathing. Witnesses have said no such equipment was in Jackson's bedroom.

"It's a very dangerous drug," said Dr. Mark Schlesinger, head of anesthesiology at Hackensack University Medical Hospital in New Jersey, who said he has administered it thousands of times in his 25-year career. "The difference between unconsciousness and no longer breathing is a very slim margin of safety."

In the hands of a trained anesthesiologist, he added: "It's a wonderful drug and a very safe drug. It's not the drug that kills somebody. It's the way it is used."

Murray was not the first doctor to give Jackson the drug as a sleep aid, although others have not been publicly named.

Jackson's reliance on propofol was first disclosed to The Associated Press days after Jackson's death. Cherilyn Lee, a nurse nutritionist who treated Jackson with vitamins, said he pleaded unsuccessfully with her to get the drug he knew as Diprivan. He said his doctor told him it was safe and he described falling asleep as soon as the drug dripped from an IV into his vein.

"I said, 'Michael, the only problem with you taking this medication,' and I had a chill in my body and tears in my eyes ... , "'You're going to take it and you're not going to wake up.'"

Lee, who said she treated him from January to April 2009, is on the trial witness list. She said she never saw Jackson use other drugs.

"He wasn't looking to get high or feel good and sedated from drugs," she said. "This was a person who was not on drugs. This was a person who was seeking help, desperately, to get some sleep, to get some rest."

Murray, who had been consulted by Jackson in his Las Vegas office, was promised $150,000 a month when he signed on as Jackson's personal physician six weeks before he died. He closed the doors of his offices there and in Houston to devote himself to the singer.

An autopsy report showed that Jackson was generally healthy, indicating his key problem was insomnia. Murray has told police he administered other drugs known as benzodiazepines, which are also used as sleep aids. But when they did not work, Jackson demanded propofol.

Murray told police he was trying to wean him from the drug and gave him a minimal dosage, then left the room for five minutes to use the bathroom. However, cell phone records suggest he was making phone calls for a longer time. His actions after he found Jackson not breathing are also central to the case.

In the two years since Jackson died, doctors have been known to refer to propofol as "The Michael Jackson drug." Some fear a backlash for its use.

Dr. Lawrence Koblinksy, head of forensic science at John Jay College in New York, said the situation of a famous man demanding propofol is an anomaly and should not cast doubt on its effectiveness in hospitals.

"If you have a drug that is wonderful, just because a celebrity dies doesn't mean you take it off the market," he said.