Drug-company scientists and executives complain that when balancing risks and rewards, the bar is often too high for drugs that are effective for many people but toxic for a few.

One executive, an expert in vaccines, once told me that scientists had come up with a way to inoculate people against H.I.V. but have declined to go public because the vaccines will kill five percent of those who take it.

"Is this worth it to the other 95 percent?" he asked, adding that he wasn't sure how he felt about such an equation. "What if it was one percent?" he mused.

Vaccines already in use cause a relatively small number of deaths each year; that is considered an acceptable risk given the alternative of widespread viral illness.

Other familiar drugs cause deaths now and then as well, including even aspirin if susceptible people take too much. Yet no one talks about removing aspirin from shelves.

Society also accepts much larger numbers of fatalities and injuries while using other products, such as automobiles, which annually kill more than 40,000 people.

Yet for those whose loved ones die because of a drug, any risk of death is too great, of course.

One way to improve drug outcomes is to better define who is susceptible to harm. This is one of the issues with Vioxx, which apparently was safe for many people to use that didn't have a heart condition.

There is evidence Merck was aware of the danger its drug posed to a subset of those taking it; in any case, the company did not follow through on that information, which might have shrunk its market—and profits.

In the wake of the Vioxx fiasco, the dilemma of what risk is too much prompted Congress to require that the F.D.A. more closely monitor drugs after approval. Some believe a fear of repeating that mistake is one reason that regulators are approving so few drugs. Last year, the F.D.A. approved only 17 new drugs, a near-record low.

So we will wait to see which way Byetta goes. Will these six deaths in three years be anomalies, or will others succumb? Have there already been too many deaths for a drug that may be better than others for some, but not dramatically? Or is the trade-off so far worth it?