Dawson said the level of Ambien found in Michele MacNeill's blood would indicate the drug was taken within a few hours of her death, and would not be in her system if she had taken it the night before, as is typical to treat insomnia.
The "cocktail" of drugs and accompanying effects, according to Dawson, "would likely render the victim unable to respond appropriately to her environment, including potential threats to her safety."
He added that the 50-year-old Pleasant Grove woman was "drug naive," likening her ability to tolerate drugs to a non-drinker being more affected by a serving of alcohol than someone who is a social drinker.
Utah's chief medical examiner, Todd Grey, testified Tuesday that "there was a potential that this mix of drugs could lead to lethal effects or sedating effects."
Prosecutors in August of this year charged Martin MacNeill, a 56-year-old doctor, with first-degree felony murder and second-degree felony obstruction of justice, claiming MacNeill wanted his wife dead so he could continue an affair with another woman, Gypsy Willis, who is expected to testify Wednesday.
But the medical examiner's office has never ruled Michele MacNeill's death a homicide.
After an autopsy in 2007, assistant medical examiner Maureen Frikke who died in 2008 ruled the manner of Michele MacNeill's death was "natural," the result of "chronic hypertension and myocarditis, which are capable of causing acute unexpected arrhythmia and sudden death."
But investigators say MacNeill called Frikke multiple times, giving misleading information that included how her body was found on April 11, 2007.
In 2010, in a new investigative report, Grey changed the cause of death to the combined effects of heart disease and drug toxicity. The manner of death was changed to "undetermined."
Grey said there was some evidence that Michele MacNeill may have drowned, pointing to prior testimony from medical responders who said that while undergoing CPR, the woman coughed up several cups of water, some of it pink and frothy.
That may be consistent with water and air found in the lungs, Grey testified, but said he did not see the microscopic findings in the lungs that indicates a drowning occurred.
"I didn't find enough physical evidence compelling enough to say, 'Yes, I know this was a drowning,' " he testified.
In other testimony, Steve Mickelson, a co-worker of Martin MacNeill's at the Utah State Developmental Center, said always thought MacNeill was somewhat eccentric, but the man's behavior in the days following his wife's death puzzled him.
MacNeill returned to work sooner than would be expected, after only four days.
He seemed calm.
And he had a new wedding band, Mickelson noticed.
Instead of the gold band that Mickelson, a nurse practitioner, usually saw on MacNeill's left hand, MacNeill was wearing a new, mostly black band.
He said he asked MacNeill about it, but it was dismissed as an old piece of jewelry he never wore.
Mickelson testified that when he asked MacNeill about his wife's death, MacNeill said she was face-down, the "wrong way," in the tub. He allegedly told his co-worker that he thought his wife may have fallen and hit her head, then drowned in the bathtub.
Last week, MacNeill's daughter, Alexis Somers, testified that her father told her a similar story, that her mother was found face-down, with her legs sticking out.
But a number of people, including neighbors and the younger daughter who found Michele MacNeill that day, testified last week that the woman was lying on her back, her body fully inside the bathtub, when they first saw her.
Somers also testified that her father overmedicated her mother after Michele MacNeill was released from the hospital following a facelift procedure.
Somers said that she had flown in from Nevada to care for her mother after her surgery on April 3, 2007.
But three days later, after Martin MacNeill had insisted on caring for his wife, the woman "was completely out of it," Somers testified. "She was very sedated. I tried to wake her up and she wouldn't wake up."
When she asked her father why her mother wouldn't wake up, he admitted that he may have overmedicated her.
Somers said that when her mother woke up in the early evening that night, she was fearful.
"She said, 'Alexis, I don't know why, but your dad kept giving me pills, giving me medication,' " according to Somers. " 'I started to throw up and he keep giving me pills, saying take this, take this.' "