A controversial animal drug, fed to a majority of pigs raised in the United States, has become the focus of a long-running trade dispute centered on meat exports.

Ractopamine hydrochloride – used to keep swine lean and boost their growth in the last weeks before slaughter – is administered to an estimated 60 to 80 percent of pigs raised in the United States. But key trading partners, including the European Union, China and Taiwan, want assurance that it is safe, and have a zero-tolerance policy for meats with even traces of the substance. So export markets are limited, according to a report published Wednesday by msnbc.com. 

The feed additive has also sparked more deaths and illnesses among pigs than any other livestock drug on the market. More than 160,000 pigs were reported to have suffered ill effects since it was introduced, according to the analysis in Business on msnbc.com.

Ractopamine, part of the beta-agonist class of drugs, quickens an animal’s heart rate and relaxes its blood vessels, leading to the growth of bigger, leaner muscles on less feed.

Pigs who are given the drug in their last weeks of life produce approximately 10 percent more meat, an increase that raises profits by $2 per head, according to the drug’s manufacturer Elanco, a branch of Eli Lilly. 

Animals process ractopamine rapidly after ingestion, excreting around 85 percent of it within a day. However, low levels of the drug can still be detected over a week after an animal has consumed the drug.

The U.S. Food and Drug Administration (FDA) determined that ractopamine was safe for use in pigs 13 years ago – in 1999 – and has since approved it for use in cattle and turkeys as well. 

The agency set a threshold for residues of the drug. Sale of any meat with levels below this amount are legal — and no meat in the United States has been found with ractopamine above the acceptable tolerances. 

But testing for ractopamine in meat products has been limited. In 2010, for example, 712 samples were taken from 26 billion pounds of beef, and the results of this testing have not yet been released.

Countries such as China and Taiwan have no threshold for ractopamine, so pork from the US with even trace amounts has been turned away.

The issue has strained the US-Taiwan trade relationship, according to the report. Taiwan began testing for the drug in meat imports last year, and pulled US beef and pork from shelves after finding traces of it.

“US trade officials are now pressing more countries to accept meat from animals raised on ractopamine — a move opposed by China and the EU,” wrote Helena Bottemiller in the investigative report. “Resolving the impasse is a top agricultural trade priority for the Obama administration, which is trying to boost exports and help revive the economy, trade officials say.” 

Canada and 24 other countries have approved the drug. But China and the EU are two big roadblocks. Together they account for 70 percent of the world’s pork production and consumption, and they appear ardently opposed to accepting meat raised on the drug. 

 “The main problem for us is that the safety of the product could not be supported with the data,” said Claudia Roncancio-Peña, a scientist who led the European food safety panel that studied the drug and criticized the science backing its safety.

Some U.S. food companies also avoid meat produced with the feed additive, including Chipotle restaurants, meat producer Niman Ranch and Whole Foods Markets.

So far, however, there is no clear consensus on whether the drug adversely affects humans. 

“There’s very little data on the low levels of veterinary drugs people are exposed to,” said Keeve Nachman, a scientist who directs the Farming for the Future program at the Johns Hopkins School of Public Health in Baltimore. “We don’t know much about the toxological significance of these exposures, and no one is really looking.”

The report published in the msnbc.com business section was written by Bottemiller (who reports here at Food Safety News), and produced by the Food & Environment Reporting Network, a new non-profit dedicated to investigative reporting on food, agriculture and environmental health.

“We tapped Helena to work independently as a freelancer for us because she’s done such great reporting on the FDA and food safety issues in general,” said Food & Environment Reporting Network editor- in-chief Samuel Fromartz. “So we thank Food Safety News for letting us borrow her talents.”

For more on the background of the dispute on this controversial feed additive and residue testing, see:

Behind the Global Fight Over Livestock Drug

More on drug residue domestic testing

Note: This article was updated to reflect a Feb. 22 clarification issued by msnbc.com that made clear that the adverse drug effects for ractopamine were reported to the FDA. The story adds that the FDA says such data do not establish that the drug caused these effects. In March, FDA said the original 218,000 ADE figure was too high because it included reports of ineffectiveness, meat abnormalities and fertility abnormalities. FDA said the number of pigs with reports of adverse effects was 160,917, so the number above has been updated.