John Mullarkey told a psychiatrist he was greeted with a hug and a kiss when he visited on-and-off girlfriend Demi Cuccia on Aug. 15, 2007 at her Monroeville home.

But then, he told Dr. Robert Wettstein, who testified yesterday at Mr. Mullarkey's homicide trial, the evening took a sinister turn.

Dr. Wettstein said Mr. Mullarkey didn't reveal any provocation on his part, but said 16-year-old Demi started screaming that she was sick of him and wanted him to leave. Dr. Wettstein said Mr. Mullarkey told him that made him feel "low, worthless, garbage, crushed, like I wasn't a person."

He told the psychiatrist that he knelt on top of Demi while she was seated on the couch, and she punched him. Mr. Mullarkey, 20, of Monroeville, vaguely remembered pulling out a 3.5-inch pocket knife he carried around.

"He couldn't recall having stabbed her," Dr. Wettstein said.

The Gateway High School cheerleader had 16 stab wounds when she was taken to the morgue. Mr. Mullarkey nearly joined her, as he sliced his own throat from ear to ear.

He sat for a third day of testimony yesterday before Common Pleas Judge Jeffrey A. Manning, as defense attorney Robert E. Stewart presented his case.

Dr. Wettstein diagnosed Mr. Mullarkey with depression and said that his cognitive abilities were impaired at the time of the stabbing. Mr. Mullarkey's defense is arguing that he was impaired to the point that he could not form the intent to kill -- a crucial component to first-degree murder.

A cause of the impairment, Mr. Stewart said in his opening statement, was the acne drug Accutane, which Mr. Mullarkey stopped taking just a couple of days before the killing. The drug has been blamed for instances of depression and suicide, though under cross-examination from Deputy District Attorney Mark V. Tranquilli, Dr. Wettstein said a review of the medical literature shows no definitive link.

To discuss the possible effects of Accutane, Mr. Stewart called pharmacist Daniel Wagner, owner of NutriFarmacy in Hampton. Dr. Wagner said he doesn't have extensive knowledge of Accutane but he said stopping any drug "cold turkey" can wreak psychological havoc.

Mr. Tranquilli -- who spent much of his cross-examination poking holes in Dr. Wagner's credentials as an expert -- pointed out that every Accutane patient goes off the medication "cold turkey" when treatment ends.

This case is the first known homicide to go to trial with Accutane used as part of the defense.

The trial resumes today.