WASHINGTON – Federal regulators say a drug from Pfizer and Ligand Pharmaceuticals effectively treats osteoporosis, though they are concerned about deaths, blood clots and other problems seen in company studies.

The Food and Drug Administration noted Thursday that women treated with Pfizer's Fablyn were more likely to die of cancer and stroke than those taking placebo.

Pfizer, the world's largest drugmaker, is seeking approval to market Fablyn to postmenopausal women with osteoporosis, which can cause debilitating bone fractures and thinning.

The FDA posted its review of the drug online Thursday ahead of a meeting Monday where its outside advisers will weigh in on the drug.

Agency reviewers noted Pfizer's drug showed positive results for preventing fractures in a study of over 8,500 postmenopausal women between ages 60 and 80. The percentage of patients who developed new fractures while taking Pfizer's drug was 3.8 percent, compared with 6.4 percent of those taking placebo.

Despite the drug's effectiveness, the FDA expressed concerns about higher rates of death and blood clots among women taking a lower dose of the drug, which regulates the feminine hormone estrogen. Reviewers also noted increased rates of gynecological problems, including vaginal bleeding and abnormal growths in the uterus.

The FDA said it would ask its panel of advisers whether these safety concerns outweigh the benefits of Pfizer's drug. The agency is not required to follow the panel's advice, though it often does.

New York-based Pfizer said the higher death rate cited by the FDA was due to an unusually low death rate among one group of patients taking placebo. The majority of Pfizer's data showed deaths were comparable among patients taking the drug and placebo, according to company briefing documents posted online Thursday.

Company spokesman Jack Cox said in an e-mail that Fablyn “offers unique benefits in the treatment of osteoporosis in postmenopausal women at increased risk of fracture.”

Government regulators have been extra cautious when reviewing drugs to treat menopausal symptoms following problems seen with hormone-replacement therapy. In 2002, a government study of over 16,000 women was halted after showing that those taking hormones had more breast cancers, heart attacks and related problems than nonusers.

Pfizer developed Fablyn using technology from Ligand Pharmaceuticals. If the FDA approves the drug, Pfizer will hold worldwide marketing rights and San Diego-based Ligand will receive royalty payments and an initial cash award.

Shares of Ligand Pharmaceuticals Inc. fell 2 cents to $3.32 in afternoon trading, while Pfizer Inc. dipped 36 cents to $18.84.