Pfizer Inc's trial data on Chantix, a drug to help people quit smoking, failed to impress U.S. Food and Drug Administration scientists, in a blow to the company's attempts to have a serious warning removed from the drug's label.

The FDA staff, in a preliminary review, expressed concerns about the collection and interpretation of data from a post-marketing study on the controversial drug.

Pfizer has been trying to have the "black box" warning - which warns of psychiatric risks including suicidal thoughts, hostility and agitation - removed from the drug's label.

In March 2015, the FDA left the warning on the label, following the recommendation of an advisory committee to wait for the outcome of the study in more than 8,000 adult smokers.

Results from the study, which compared Chantix or GlaxoSmithKline Plc's Zyban with a placebo or a nicotine patch in smokers with and without a history of psychiatric disorders, showed that the drug did not significantly increase the incidence of serious neuropsychiatric side-effects.

However, FDA staff disputed the results, flagging inconsistencies in data collection and characterization of the severity of some side-effects.

These factors could have led to biased results, they said, ahead of a meeting of independent advisers on Wednesday.

The advisers will discuss the trial findings and decide whether the warning should be removed. The FDA is not obligated to follow the panel's recommendations, but it usually does.

James Rusnak, chief development officer of Pfizer's cardiovascular metabolic unit, said there were bound to be variations in investigators' judgment within the boundaries of the trial protocol, just like in the case of doctors in clinical practice.

"Ultimately, what was reported as an adverse event, was determined based upon the investigator's local determination of the subject presenting in front of them," he said.

Any subjectivity in investigator's judgment would be spread across the four treatment groups and not be limited to Chantix, Rusnak noted.

In May, European regulators lifted a warning on the drug, sold under the brand name Champix in Europe, based on the results of a large study.

Investors were betting big on the drug when Pfizer launched it in 2006, but reports of mental health problems in users led to the FDA imposing the black box warning three years later.

Chantix, which loses patent exclusivity in four years, generated global sales of $671 million in 2015. (Reporting by Natalie Grover in Bengaluru and Toni Clarke in Washington; Editing by Kirti Pandey)