ABC News’ Jim Avila and Bryan Lavietes report:

An expert on the anesthetic that killed Michael Jackson testified today that  ingesting propofol orally is not fatal.

The prosecution’s final witness, Dr. Steven Shafer from Columbia University, testified that he personally conducted a clinical study that demonstrated that propofol is not absorbed through the stomach and that it is ineffective as an anesthetic when taken orally.

Shafer said it is not fatal when ingested through the mouth as the liver and stomach lining filters the potency from the drug.

Dr. Conrad Murray looks on during his involuntary manslaughter trial in Los Angeles, Oct. 20, 2011. (Reed Saxon-Pool/AP Photo)

The prosecution hammered away at the defense’s contention that Jackson killed himself by self-injecting propofol.

Assistant District Attorney Walgren asked Shafer, “There was zero possibility this was the cause of Michael Jackson’s death?”

“Correct” answered Shafer.

Shafer said the only workable scenario he can work out that explains how Jackson died is that he was on an IV drip of propofol and did not separately inject himself with the drug.

Earlier, Shafer testified that given the condition of Michael Jackson’s veins and that he was already sedated by Murray, it would be highly be unlikely the singer could wake up and Murray could somehow find a useable vein.

“It is very clearly explained in Dr. Murray’s interview that Michael Jackson did not have good veins, in fact they were so poor he had to start this IV in a vein below the left knee.” Shafer said. “So the possibility of direct self injection seems extremely unlikely…you would not want to inject into a vein and miss because that would be extremely painful.”

Intense cross-examination of Shafer by the defense could last well into Friday and the defense is expected to present its case.

It is expected to last at least a week.

The defense’s plan includes presenting 15 witnesses to testify on Murray’s character and his side of the medical case.