A side effect of an acne medication caused a Pittsburgh-area man to stab his cheerleader girlfriend to death, his lawyer argued in court.

John Mullarkey's attorney said at his murder trial that his client was under the influence of Accutane, a drug that carries a warning that it can cause depression and even suicide, when he stabbed 16-year-old Demi Cuccia repeatedly in 2007.

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

The Pittsburgh Post-Gazette said Wednesday that Stewart proposed that Mullarkey, 20, of Monroeville be convicted on a lesser count of third-degree murder on the grounds his mental condition prevented him from forming the intent to kill.

Prosecutor Mark Tranquilli scoffed at the idea that a "pimple pill" would turn someone homicidal and said the intent to kill has most likely existed at one point while the victim was being stabbed 16 times.