In no way, according to defense attorney Robert E. Stewart, is he trying to excuse John Mullarkey of responsibility.

Stabbing his on-and-off girlfriend, Gateway High School cheerleader Demi Cuccia, 16 times was a horrific crime, but Mr. Stewart asked jurors to delve into the mind of the lovesick Monroeville man to determine whether he deserves a first-degree murder conviction.

And Mr. Mullarkey's mind, Mr. Stewart said as the trial opened yesterday, was thrown out of whack by an acne drug.

"I am not standing here and saying that this is it -- the Accutane did it," Mr. Stewart said in his opening statement. "No, that is one component."

Mr. Stewart could find no precedent for Accutane being used as a murder defense.

But he's hoping jurors determine that Mr. Mullarkey, 20, was unable to form the specific intent to kill partly because of Accutane's effects, and convict him of third-degree murder. First-degree carries a mandatory life sentence, while third has a maximum 20- to 40-year sentence.

Dressed in a dark blue vest, a light blue dress shirt and brown slacks, the youthful looking Mr. Mullarkey stared into a corner of the courtroom for most of the afternoon.

Deputy District Attorney Mark V. Tranquilli, in his opening, scoffed at the idea that a "pimple pill" could cause a homicide.

"John Mullarkey wants to redirect the blame from himself -- the man who wielded the knife -- to a faceless prescriptive drug that cures pimples," Mr. Tranquilli said.

The prosecutor presented four witnesses yesterday who gave their recollections of Aug. 15, 2007, when the late afternoon was punctured by 16-year-old Demi's screams.

Gayle Slomer came out of a neighboring townhouse, off of Elliott Road in Monroeville, and the bloody and shrieking Demi fell into her arms. Shortly after, Mr. Mullarkey emerged from Demi's house with a self-inflicted slash across his neck.

"Get away from me; I hate you," Demi screamed, according to Ms. Slomer.

By the time police and paramedics arrived, Demi couldn't speak. She died less than an hour later at Forbes Regional Hospital.

Monroeville police Lt. Lawrence Lyons testified that when he arrived at the scene, Mr. Mullarkey was sprawled on the ground outside Demi's home, unable to speak. When Lt. Lyons asked Mr. Mullarkey if the knife was inside the house, he nodded yes.

Mr. Stewart, who stressed the severity of Mr. Mullarkey's wounds in his cross-examination, said in his opening that the couple got to know each other because Mr. Mullarkey was friends with Demi's older brother. They would split up and date others from time to time before getting back together.

In the spring of 2007, shortly before graduating from Gateway, Mr. Mullarkey started taking Accutane, Mr. Stewart said. The drug, first introduced in 1982, has been blamed for incidences of depression and suicide -- prompting warnings on the label.

Mr. Stewart said just a couple days before the killing, Mr. Mullarkey abruptly stopped taking the drug because he didn't like the way he was acting -- including a crying spell and a harsh exchange of words with a new guy Demi was dating.

He went over to Demi's house Aug. 15 thinking they were going to get back together, Mr. Stewart said.

"Something went horribly wrong," Mr. Stewart said. "He doesn't know why. You will hear experts testify that he has no recollection of the actual event itself."

Mr. Tranquilli said he will present a series of text messages between the two in the hours and minutes leading up to the killing to show the state of the relationship, but he posited to the jury that the act itself displays the characteristics of first-degree murder.

"When you decide to stab someone 16 times," Mr. Tranquilli said, making a stabbing motion with his right arm for emphasis, "somewhere along the continuum of stabs, you form the intent to kill."