A Quebec psychiatrist testifying at Francis Proulx's murder trial said the defendant did not exhibit any abnormal reaction to an anti-depressant he was taking when the crime was committed.

Proulx is facing a jury trial on one first-degree murder charge in the slaying of political aide Nancy Michaud, who was found dead in a basement in May 2008 after she went missing from her family home in Rivière-Ouelle.

He has openly confessed to killing Michaud — in a videotaped statement he gave while in police custody, and again in court — but has pleaded not guilty by reason of a "mental disorder."

Proulx's lawyer has spent weeks building a defence based on the contention that his client suffered rare side-effects from taking venlafaxine, a popular anti-depressant medication known as Effexor.

On Tuesday, psychiatrist Dr. Sylvain Faucher told the Quebec City court that Proulx appeared to be a traumatized and marginalized person who had trouble integrating in the world — but that Effexor did not radically change his behaviour.

Faucher testified that Proulx did not appear distracted or agitated, nor did he suffer from insomnia or euphoria, all symptoms of manic behaviour that is a rare side effect of venlafaxine.

Proulx appears to be someone who tried to redefine himself as a "winner" instead of a "loser," Faucher said, and his past criminal behaviour suggests he may have felt triumph over his bad childhood, and more recent losses.

Crown prosecutors called Faucher as a rebuttal witness against the defence's experts — two psychiatrists and a psychoneuropharmacologist — who told the jury earlier this month that Effexor made Proulx abnormally impulsive and impeded his judgment.

Michaud, 37, was an assistant to Quebec Natural Resources Minister Claude Béchard at the time of her death.