Pfizer officials counter that even though the study concludes that it is safe to take the drug for 52 weeks straight, the package labeling suggests a course of 12 weeks to help smokers quit, followed by another 12-week course to help patients continue their abstinence.
"It is very important to separate what's in the scientific literature from what's in the label," said Dr. Ponni Subbiah, Pfizer's vice president of medical affairs. "Where [doctors] should get prescribing information is from the label."
Pfizer, like other drug companies, pays doctors to give talks and study their products. ABC News contacted two of these doctors, both of whom defended Chantix.
Mike Cummings, director of the Department of Cancer Prevention at Roswell Park Cancer Institute in Buffalo, N.Y., called Spangler's response to the Pfizer study "alarmist."
"Dr. Spangler's recommendation that varenicline should be restricted until additional objective safety data are available seems extreme to me," says Cummings.
Another doctor who received money from Pfizer, in this case to study the drug, said he sees no concerns about the safety of varenicline when prescribed according to the label. "I try to keep up with this stuff because I prescribe [Chantix]. I have not seen anything that makes me feel like something is going on here," said Dr. Joseph DiFranza, professor of family medicine and community health at the University of Massachusetts Medical School in Worcester.
The authors of the ISMP report, however, say that Spangler's concerns about Chantix should have been viewed as a "red flag" that safety questions surrounding the drug needed additional attention.
"We found a dramatic increase in the number of reported serious adverse events with Chantix," said Dr. Curt Furberg, professor of public health sciences at Wake Forest and a co-author of the ISMP report.
And even DiFranza admits that the Pfizer study published last year may have been too small and too underpowered to affirm the safety of Chantix when taken for a year.
"What [Spangler's response] says is that the study was underpowered," DiFranza says. "It means the study's too small … That's a valid point.
"I would think that the FDA would look at this and demand a larger study before approving Chantix for long-term use; I would be surprised if they were given the go-ahead to market Chantix for 12 months based on this study."
Yet, to date, the FDA has demanded no such study. Pfizer has not yet sought FDA approval to change its label to allow long-term use. But at least eight subsequent studies on Chantix — appearing in such high-level medical journals as Annals of Internal Medicine and the Proceedings of the National Academy of Sciences — have cited this study by Pfizer in support of the drug's safety for long-term use.
Varenicline, which received FDA approval in May 2006 after being granted a priority review, has been touted as one of the most effective medicines available today to help smokers kick the habit.
Chantix helped 33 percent of smokers who used it avoid tobacco use for six months after quitting, according to a federal review of previous studies. By comparison, only 14 percent of smokers taking placebo pills were able to abstain from tobacco for as long.
The safety of the drug came into question with last month's release of the ISMP report, which found that by the last quarter of 2007, the FDA received 988 reports of serious injuries in those taking the drug.