WASHINGTON (Reuters) - Hundreds of patients taking Pfizer Inc.'s anti-smoking drug Chantix have reported serious accidents, vision problems and heart trouble, researchers said yesterday, sending shares of the world's largest drugmaker to their lowest level since 1997.

US 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 non-profit 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 US 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 yesterday following release of the study, having closed on the New York Stock Exchange down four cents at $20.01.