The agency said a study linked the drug to mental confusion and other problems in people operating vehicles, which could put passengers at risk, USA Today reported Thursday.

The study was posted online by the Institute for Safe Medication Practices. Co-author Curt Furberg, a Wake Forest University medical epidemiologist, said 5 million people worldwide -- 3.5 million in the United States -- have used since Chantix since it was approved in May 2006

In February, the U.S. Food and Drug Administration issued an alert about label revisions to warn of depressed moods, suicidal thoughts and suicide linked to the drug. On Friday, the FDA approved a medication guide that pharmacists will distribute.

"We agree that these findings are a signal that requires further investigation and confirmation," FDA spokeswoman Susan Cruzan said.

"This is not any new data," Martina Flammer, drugmaker Pfizer's senior medical director for Chantix, told USA Today. She said it wasn't clear whether problems Chantix users experienced were because of the drug, withdrawal or something else.

Pfizer will monitor and study Chantix for "many, many years," she said.