The arthritis pill Vioxx was withdrawn but menopause hormones were not, even though both were tied to heart risks. A multiple sclerosis medicine was pulled and later allowed back on.

So, when is a drug too risky to stay on the market?

Drug safety questions arose again this week, as calls mount for the diabetes pill Avandia to be withdrawn. Surprisingly, the Food and Drug Administration has no firm rules for deciding such cases — just a murky guideline of “when the risks exceed the benefits.”

“Each drug has its own complex story,” so comparisons to previous decisions can’t be made, said Dr. Joshua Sharfstein, the FDA’s principal deputy commissioner.

The agency does need better criteria for weighing drug safety, he said. It has asked a group of outside scientists, the Institute of Medicine, to give advice. A report is expected before the July 13-14 hearing on Avandia, a controversial pill whose maker, GlaxoSmithKline PLC, insists is safe.

The FDA can order a drug off the market, but that can be challenged in court. Usually, a company voluntarily withdraws the medicine at the FDA’s request.

Many things influence whether such a request is made, said Dr. Brian Strom, a drug safety expert at the University of Pennsylvania. He is a longtime FDA drug safety adviser who has consulted for Takeda Pharmaceuticals, which makes Actos, an Avandia rival.

Some factors to consider:

• How serious is the illness being treated? Severe side effects are accepted for cancer drugs, for example, but not for an allergy drug such as Seldane, which on rare occasions caused sudden death and was withdrawn a decade ago, Strom said.

• How big is the harm? “Causing a little nausea isn’t so bad. Killing people is,” Strom said.

• How frequent are the risks versus the benefits?

A large federal study was stopped in 2002 after researchers saw more breast cancers and heart problems among women taking estrogen-progestin pills. Yet the absolute risk of suffering one of these problems was relatively small, and hormones remain the most effective treatment for menopause symptoms.

The multiple sclerosis drug Tysabri was withdrawn in 2005 because of a rare but serious side effect, then returned to the market a year later under a restricted distribution system. There are few drugs available to help people with that disease.

• Are there safer alternatives? This may prove to be the strongest argument for those wanting Avandia withdrawn. A new study this week found Actos — the only other drug that works the way Avandia does — to be safer.