The Food and Drug Administration told the companies that the data was not strong enough to support approval of the Prochieve drug and that they needed to do additional clinical studies, Watson and Columbia said.

"We have formally requested an end-of-review meeting with FDA to determine if a viable path forward can be established for this application," Watson Chief Executive Officer Paul Bisaro said in a statement.

The decision was not a major surprise after an advisory panel to the FDA recommended in January that the agency reject the drug, already used in some other countries, because there was not enough data to show it worked.

The vaginal gel, which contains the hormone progesterone, is meant for women with a short cervix, who have a higher risk of premature birth.

Preterm birth affects 10 percent to 12 percent of all pregnancies in the United States, and babies born early have a higher risk of early death and long-term health and developmental problems.

A similar drug called Makena, from K-V Pharmaceutical Co, is already approved in the United States to prevent premature birth in women who have a history with the condition. But there is currently no other drug approved for women with a short cervix, who may be at higher risk of preterm birth but have never had problems before.

In a preliminary review of the gel, FDA staff said Prochieve did not work as well for women in the United States, and the drug's overall success may have been influenced by high performance in countries such as Belarus and South Africa.

The rejection is a setback for Watson, which is one of the world's biggest generic drugmakers and is also developing a brand-name pharmaceutical franchise in men's and women's health products.

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