Pfizer’s shares dropped more than 2% yesterday after US regulators cited concerns over the drug giant’s osteoporosis drug Fablyn (lasofoxifene).

New research has shown women who took the drug had a higher incidence of blood clots and a higher proportion of deaths in the group taking the lower dose compared with those on placebo. The damming news comes just days before an advisory committee reviews the clinical data and makes a decision whether the drug should be approved.

The drug, which is developed using technology from Ligand Pharmaceuticals, has had a rocky road thus far. In 2005 and 2006, Pfizer received not approvals letters related to a theoretical risk of endometrial cancer and concerns about increased risks of blood clots and stroke.

In response, the pharma companies went back to the drawing board and undertook a five-year multi-national study involving more than 9,000 women to dispel the concerns.

In the FDA review of the new results, published online yesterday, there was no indication of increased endometrial cancer and the FDA acknowledged that Fablyn was effective in treating osteoporosis in post-menopausal women – only 3.8% of patients developed new fractures when taking the drug compared with 6.4% on placebo.

However, the FDA was concerned with results from the lower-dose group that showed higher rates of blood clots and deaths. “The higher proportion of deaths in the 0.25-milligram lasofoxifene group was statistically significant compared to that in the placebo group, based on five-year data,” the review said.

According to the research, there were 38% more deaths in the lower-dose group than the placebo, and a total of 228 deaths among all patients taking Fablyn. Deaths from cancer and stroke were drastically increased in the drug-taking group compared with the placebo.

Following problems with hormone-replacement therapy, the FDA is being extra vigilant when it comes to reviewing drugs to treat menopausal symptoms. If approved, analysts have estimated that Fablyn, a selective estrogen receptor modulator, could bring in as much as $500 million a year in sales.