Hundreds of patients taking Pfizer Inc's anti-smoking drug Chantix have reported serious accidents, vision problems and heart trouble, researchers said on Wednesday, sending shares of the world's largest drugmaker to their lowest level since 1997.
U.S. aviation regulators responded quickly to the research, saying they would prohibit use of Chantix by private and commercial pilots, while consumer advocates called for stronger warnings on the drug's label.
Chantix, also known as varenicline, has already been linked to depression and suicide, among other problems.
Researchers at the nonprofit Institute for Safe Medication Practices, and Wake Forest University, said they found hundreds of reported problems since the drug's 2006 approval that included blurred vision, dizziness, confusion and loss of consciousness.
"These data provide a strong signal that the risks of varenicline treatment have been underestimated and show that a wide spectrum of serious injuries are being reported in large numbers," the researchers said.
The data does not definitively prove that Chantix is at fault but does show a strong signal, the researchers cautioned, adding that the U.S. Food and Drug Administration and Pfizer should take further steps, including conducting more studies.
Pfizer officials defended Chantix, one of the struggling drugmaker's newest medicines, saying the side effects are already mentioned on the drug's label.
"When you've got the nicotine withdrawal along with Chantix, it's just very difficult to tell what is causing it," said Gretchen Dieck, Pfizer's senior vice president for safety and risk management.
Chantix works by targeting brain receptors affected by nicotine, tobacco's addictive ingredient. The drug blocks some of the effects of nicotine while also providing a nicotine-like buzz to help curb withdrawal.
Shares of the drugmaker fell 1.2 percent to $19.76 in after-hours trading Wednesday following release of the study, having closed on the New York Stock Exchange down 4 cents at $20.01.
Pfizer has touted Chantix as one of its biggest hopes for future earnings growth, but its sales have been hurt in recent months by safety concerns.
In February, the FDA warned about the risk of mood and behavior changes with Chantix and called for new warnings on the drug's label. It also said the drug could impair patients' ability to drive or use heavy equipment.
The drug brought in $277 million in U.S. sales during the first quarter of 2008, but has since seen its prescriptions fall 5 percent, according to Bear Stearns analyst John Boris. On Monday he revised his estimate for 2008 sales to $640 million, down from $915 million.
But Morningstar analyst Damien Conover said the new data was not surprising, given that many smokers have multiple health problems.
Conover said many smokers might accept the risks for a better chance to quit smoking, which can cause lung cancer and other health problems. He said Chantix should still bring in global sales this year of $1 billion.
FDA spokeswoman Susan Cruzan said the FDA was still investigating the drug's psychiatric effects. Wider reviews of new drugs like Chantix were desirable but "FDA currently does not have adequate staffing to conduct such reviews."
The same brain receptors targeted by Chantix also control muscle movement, according to the researchers, who combed FDA data on reported side effects.