The abortion pill remains safe enough to stay on the market, according to the FDA. The rate of sepsis is about 1 in 100,000 uses of the Mifeprex system, comparable to infection risks with surgical abortions and childbirth.

At least five US women have died after taking the pill since it began selling in 2000, although the Food and Drug Administration stressed that it could not prove the drug was to blame.

But the four deaths caused by bloodstream infections, or sepsis, all occurred in women who didn't follow FDA-approved instructions for a pill-triggered abortion, said agency drug chief Dr. Steven Galson.

"We don't know that this off-label use has caused the deaths," he cautioned.

May Not Have Typical Symptoms

Still, the FDA issued a public health advisory warning doctors of the possible link and urging them to be on the lookout for infections in women who have used the Mifeprex abortion pill.

Such women may not have typical symptoms of sepsis, such as a fever, Galson stressed.

Tuesday's action comes just eight months after the FDA warned about two sepsis cases associated with Mifeprex, also called RU-486 or mifepristone. Additional sepsis cases were reported to the agency in April and June.

The drug, sold by Danco Laboratories, is approved to terminate pregnancy up to 49 days after the beginning of the last menstrual cycle. It blocks a hormone required to sustain a pregnancy. When followed by another medicine, misoprostol, the pregnancy is terminated.

Off-Label Use

The FDA's instructions call for women to swallow both pills, but most abortion clinics instead instruct that the misoprostol tablet be inserted into the vagina, Galson said. Studies have shown it can work that way, too.

But the four sepsis deaths, all reported from California, came after this so-called off-label use. Galson couldn't say if the women had inserted the tablet vaginally themselves.

In two of the infections, doctors identified a bacterium called Clostridium sordelli, a common germ not usually associated with illness, he said.

The abortion pill remains safe enough to stay on the market, Galson stressed. The rate of sepsis is about 1 in 100,000 uses of the Mifeprex system, comparable to infection risks with surgical abortions and childbirth.

"There are no alarm bells going off with this rate," he said.

The other US death associated with Mifeprex was a case of a ruptured ectopic, or tubal, pregnancy in October 2001. The drug is not to be used by women with suspected or confirmed ectopic pregnancies, a life-threatening condition in which the fertilized egg has implanted outside the uterus.