CHICAGO—Psychiatrists say it's a common scenario—troubled patients stop taking their medicine, because of cost, side effects, the stigma, or delusions that they don't need it. The consequences can be tragic, though rarely as horrific as the Valentine's Day suicide-slaughter at Northern Illinois University.

No one knows what triggered Steven Kazmierczak's campus rampage, yet one of the clues to an emerging psychiatric profile is this: His girlfriend says he recently stopped taking Prozac.

Prozac is a drug generally prescribed for major depression. It and similar antidepressants carry warning labels about risks for suicidal behavior in patients younger than Kazmierczak, who was 27.

Still, stopping these drugs can also lead to suicidal thoughts and behavior. And taking them may increase the risk for other violence if they're mistakenly prescribed as the only treatment for patients in a depressive phase of bipolar disorder, psychiatrists say. In that case, the drugs may trigger a manic phase that could include aggressive behavior toward others.