But the centerpiece of Merck's plan, a large "field evaluation" of Zilmax, remains in limbo. This study was supposed to include up to 240,000 cattle, at a variety of commercial feedlots. Merck recruited a university researcher to carry it out, and it was supposed to begin last year.

Feedlot operators are refusing to participate, though, because they don't want to be stuck with cattle that they can't sell. And their customers, the beef processors, remain skittish. "We're not accepting cattle fed with Zilmax," says Mike Martin, from Cargill, one of four companies that dominate the beef industry. (The others are Tyson, JBS, and National Beef.)

Cargill's reluctance to take Zilmax-fed cattle, Martin says, is based in part on continued uncertainty about what caused those health problems in cattle. But he also mentioned another reason: The drug can complicate beef exports. Some countries won't accept beef from cattle that were fed Zilmax.

Kelly Goss, a spokesperson for Merck Animal Health, says that organizing the Zilmax study has "been more time intensive and complicated than we anticipated." But she says the company still hopes to proceed with it. "Our intent is not to rush this," she says.

Zilmax is part of a class of drugs called beta agonists, which are chemically similar to the human hormone adrenaline. Another beta agonist, called ractopamine, is commonly fed to pigs. They cause animals to grow more muscle.

Dan Thomson, a researcher at Kansas State University who has studied the effects of Zilmax on cattle, says that Merck has been acting responsibly in its efforts to revive sales of the drug. "I think that the changes they've made have been all for the better," he says.

Thomson also says that beta agonists such as Zilmax are not the sole cause of "fatigued cattle syndrome." Those symptoms, he says, are a reaction to stress. Beta agonists may contribute to it, but so do heat, being transported in trailers and interactions with humans. "We have been able to study it in cattle that were fed beta agonists and cattle that were not fed beta agonists," he says.

Copyright 2015 NPR.

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Beef Packers Block Plan To Revive Growth-Promoting Drug