Correction Appended

CHARLESTON, S.C., Feb. 14 - A teenager on trial for killing his grandparents when he was 12 and taking the antidepressant Zoloft can be found not guilty of murder if he was "involuntarily intoxicated" by the drug, a judge told jurors after closing arguments Monday, offering them an option to acquit him without finding him insane.

The teenager, Christopher Pittman, now 15, confessed to the killings, but his lawyers have argued that Zoloft made him manic and violent. After shooting his grandparents, Joe and Joy Pittman, in their bed, he set fire to the house and fled in their car, then claimed to have been kidnapped. Prosecutors say the boy's actions prove that he was aware that what he had done was wrong.

He later said of his grandparents, who had taken him in when he was having trouble at home, "I'm not sorry. They deserved it," according to his confession to investigators presented during the two-week trial.

Defense arguments that blame antidepressants have been used in cases with varying success, but the Pittman case has received particular attention because of the defendant's youth. Last October, three years after the death of the Pittmans and in the midst of a storm of controversy over the effects of antidepressants on children and adolescents, the Federal Drug Administration said that all antidepressants must carry a warning of an increased risk of suicidal thoughts and behavior for young patients.