*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.

Read/learn more of the AP story at CBS News.