Duramed Pharmaceuticals Inc. asked federal regulators Wednesday to force the maker of the Premarin brand of estrogen to identify the drug's active ingredients.

Federal law requires drug makers to identify active ingredients in their products.

"Simply put, you can't have it both ways," said E. Thomas Arington, Duramed's chief executive.

In May, the Food and Drug Administration denied Duramed's application to make a synthetic version of Premarin because it lacked a form of estrogen earlier deemed to be an impurity.

Wyeth-Ayerst Laboratories, the maker of Premarin, subsequently submitted evidence during FDA review of Duramed's application, that the missing ingredient, a type of estrogen called delta 8,9, might have a role in Premarin's ability to strengthen bones and counter the bone-deteriorating effects of osteoporosis.

Duramed, which spent more than $30 million developing the generic drug, has appealed the FDA ruling.

The request filed Wednesday - a citizen's petition - takes a new course.

The petition asks the FDA to withhold approval of any new uses or combinations of Premarin until the active ingredients are known. "We are presenting their case back to them," Mr. Arington said.

The goal is to pressure Wyeth Ayerst, a division of American Home Products Corp., to identify Premarin's active ingredients. "Unless somebody has a reason to resolve this, I don't see it getting done quickly," said John Rapoza, vice president of regulatory affairs for Duramed.

Wyeth-Ayerst has little incentive to move quickly to identify Premarin's active ingredients given that the uncertainty hinders attempts to make a generic version of the product.

"This petition helps us a bit, we think," Mr. Rapoza said. If the FDA grants Duramed's request, it won't lead to a quick generic version of Premarin.

"You are looking at a fairly substantial amount of time to resolve this issue," Mr. Rapoza said. "It is not going to be one year. It is not going to be two. It's going to be longer than that."

Said Audrey Ashby, spokeswoman for Wyeth Ayerst: "Wyeth Ayerst believes that there is little basis for Duramed's citizen's petition. Conjugated estrogens have (long) been indicated as the active ingredient of Premarin."

Duramed expects to win approval for several other products in the second half of this year.