The Food and Drug Administration is taking a closer look at Pfizer Inc's anti-smoking drug Chantix after reports of hundreds of patients experiencing serious problems, an agency official told Reuters on Tuesday.

Janet Woodcock, director of the FDA's Center for Drug Evaluation and Research, said the agency is reviewing all the reported cases. "We're are looking at them, but it takes awhile," she said in an interview.

The drug, which aims to help smokers quit their habit in part by mimicking some of nicotine's effects, has already been linked to psychiatric side effects such as depression and suicide.

But U.S. researchers at the nonprofit Institute for Safe Medication Practices and Wake Forest University last week found hundreds of reported cases of accidents, heart trouble, vision loss and other problems with Chantix since its 2006 approval.

FDA's Woodcock said the agency does not yet know whether the drug directly caused the complications cited in the researchers' report. Agency scientists are in the process of sorting through the cases, she added.

It was not clear how long it would take the agency to take any action. Woodcock said agency staffers would need time to do their work.

"We are not able to put the amount of resources against this we would need to do it in a very prompt matter," she said.

Pfizer officials have defended Chantix, a key drug for the drugmaker that has touted its potential to generate new revenue, but prescriptions for the drug have been declining, according to some analysts.

A representative for Pfizer could not be immediately reached for comment.

Shares of Pfizer were down more than 1 percent, or 27 cents, at $19.26 in early afternoon trading on the New York Stock Exchange.