Reports of gunman's antidepressant use renew debate over side effects.

Steven Kazmierczak stopped taking Prozac before he shot to death five Northern Illinois University students and himself, his girlfriend said Feb. 17 in a remark likely to fuel the debate over the risks and benefits of drug treatment for emotional problems, "The New York Times" reported.

Over the years, the antidepressant Prozac and its cousins, including Paxil and Zoloft, have been linked to suicide and violence in hundreds of patients. Tens of millions of people have taken them, and doctors say it is almost impossible to tell whether the spasms of violence stem in part from drug reactions or the underlying illnesses.

"It's a real chicken-and-egg sort of situation," said Jane Garland, M.D., director of the Mood and Anxiety Disorders Clinic at BC Children's Hospital in Vancouver, British Columbia.

Garland said some people could and did become agitated and unpredictable in response to the drugs, usually just after starting to take them or soon after stopping.

"But it's hard to make a case for a withdrawal reaction here, because Prozac comes out of the system gradually," she said.

The girlfriend, Jessica Baty, said in an interview on CNN that Kazmierczak took Prozac to battle anxiety and compulsive behavior but that it "made him feel like a zombie and lazy."

She said that in the days leading up to the shooting he was not behaving erratically, as university officials had suggested.

The drug labels warn about agitation and severe restlessness, and display a prominent caution that the medications increase the risk of suicidal thinking and behavior in some children and young adults.

Psychiatrists said that stopping an antidepressant could cause effects like lightheadedness, nausea and agitation as the brain adjusted, The Times reported. Among the most commonly prescribed drugs, Prozac is the least likely to cause withdrawal effects because it stays in the system longest, the doctors said.