Pfizer Inc failed to convince the U.S. Food and Drug Administration to remove a black box warning on its controversial quit-smoking drug Chantix, with an advisory panel to the agency voting against the removal on Thursday.

A majority of the panel voted to keep the health regulator's most severe warning label on the treatment, in keeping with the recommendations of FDA staff two days earlier.

The panel agreed to revisit its stance on the warning label once data from Pfizer's post-marketing study of Chantix is available.

Pfizer is currently conducting a study on 8,000 people comparing Chantix's serious neuropsychological side effects with two other smoking-cessation treatments and a placebo. Data from the study is expected in the third quarter of 2015.

The company expects this data to support Chantix's safety profile.

"I'm hoping the data confirms all the information we already have today," Steve Romano, senior vice president of Pfizer's Global Innovative Pharmaceutical Business, told Reuters on Wednesday.

Chantix, also known as varenicline, is one of Pfizer's most controversial drugs and has a number of severe side effects, including suicidal thoughts, erratic behavior and drowsiness.

The FDA placed a black box warning — its most severe and restrictive warning — on the product in 2009, highlighting the drug's adverse neuropsychological effects.

The panel pointed out that data from Pfizer's analysis of third-party observational studies did not include all of Chantix's psychological side effects.

"Another factor into how the drug is weighed is that there may potential harm not just to the person taking the drug but people around them," a panel member said.

Diana Zuckerman, President of the National Center for Health Research (NCHR), called the drug's side effects "distinctly worrisome".

The NCHR is one of five not-for-profit organizations that had filed a citizen petition asking the FDA to include risks of aggression/violence, psychosis and depression in the black box warning.

The retention of the warning label is not expected to have an impact on Pfizer's sales or profits, analysts had said. The company had 14 drugs that had better sales than Chantix in 2013.

Pfizer shares closed down 1.7 percent at $27.70 on the New York Stock Exchange on Thursday.