Not so for the families of some 25 would-be quitters in Britain, Canada and Australia who believe Zyban played a part in the sudden and unexpected deaths of their loved ones.

In England, the youngest victim was a 21-year-old flight attendant with a 10-a-day cigarette habit, who had been in good health until she was found dead on the floor of a hotel room during a stopover.

Kerry Weston had been taking Zyban less than three weeks.

Here, Zyban has been prescribed to more than 5 million smokers since it was approved by the FDA as a prescription anti-smoking drug in 1997. Previously, it was available as an anti-depressant under the brand name Wellbutrin.

Though the FDA has no concrete evidence of fatalities among American Zyban users, Dr. Jerome Giron, pulmonary specialist at the NYU Downtown hospital believes the deaths overseas should not be ignored.

"It definitely needs to be looked into thoroughly," he said. "Any death would be significant. If there's a chance it's caused by medication, it's something that should be factored into the equation."

GlaxoSmithKline, the pharmaceutical giant which manufactures the drug in Britain, says there are no proven links between its drug and the 19 fatalities there.

The causes include heart attacks, suicides, brain disorders and an asthma attack.

Yet none of these ailments are listed among the side effects of the drug, which could include seizures, convulsions or loss of consciousness for one in every 1,000 Zyban takers, according to the Food and Drug Administration.

More common are agitation, insomnia and anxiety.

"It's just not cut and dried," said Laura Bradbard, a spokeswoman for the FDA. "There are no complete answers."

Yet a number of patients seem to experience more moderate side effects. Giron has prescribed Zyban to about 100 patients, and only around a third have stuck with the drug for the full eight-week course, he said.

"I've found it's not very well tolerated. People have said that they feel ‘spaced out' and ‘jittery' like they were going out of their minds. There've been a lot of problems with nervousness."

Even amid questions, many doctors and anti-smoking advocates argue that the benefits of Zyban outweigh the dangers - especially for heavy smokers.

"Every drug has with it a risk, and what the clinicians and the patients must do is weigh that small risk against the benefits and make a judgment," said Dr. Michael Fiore of the Center for Tobacco Research and Intervention, who says half of all American smokers are killed by their habit.