The milk you drink is extremely unlikely to be contaminated with errant traces of drugs, a major new federal study has found. Even so, the findings have inspired some additional federal scrutiny of the nation’s milk supply.

As we recently reported, the dairy industry tests milk to make sure a handful of common antibiotics do not show up in our lattes and cereal bowls. But it doesn’t look for many other drugs that also are used to treat the cows for illness.

That’s despite the fact that when dairy cows are eventually slaughtered, they are far more likely than most other animals to have traces of drugs in their bodies – residues that make their meat unfit for human consumption and illegal to sell.

That’s why in 2012, the U.S. Food and Drug Administration set out to test milk for 31 drugs, most of which the dairy industry does not routinely test for. The agency examined more than 1,900 milk samples, about half of which came from dairy farms whose cows had previously shown excessive drug residues.

The FDA had been under pressure from consumer advocates to release the results of the study since it finished collecting the samples over two years ago. It finally released its findings this week: More than 99 percent of the samples were not tainted with the drugs; 15 samples tested positive for drug residues – that’s 0.8 percent. In one sample, researchers found gentamicin, an antibiotic that’s illegal to use in cattle but has been found in dairy cow carcasses as recently as November.

The dairy industry hailed the study’s findings, with Jim Mulhern, president and CEO of the National Milk Producers Federation, calling them “a positive affirmation of our milk safety in the U.S.” Still, Mulhern called for “continued education among farmers, veterinarians, and pharmaceutical companies” and “continued outreach on how to prevent trace levels of residues in the future.”

In the study, dairy farms that had a history of drug residue violations in carcasses were not significantly more likely to have traces of drugs in their milk. However, they did have drug residues from a wider variety of drugs in their milk.

The FDA did not recommend routinely testing milk for any additional drugs. But the agency has indicated that it may lean on state regulators to start sampling milk on farms where they are investigating illegal drug residues in the carcasses of dairy cows.

Under federal rules, the industry tests raw milk, both conventional and organic, for at least four of six beta-lactams, a common class of antibiotics that includes penicillin and amoxicillin.Credit: Michael Conroy/Associated Press Credit: Michael Conroy/Associated Press

Consumer advocates say that’s not enough.

David Plunkett, senior staff attorney for food safety at the Center for Science in the Public Interest, said the FDA needs to fully investigate a dairy when the cattle it sends to slaughter tests positive.

“Currently, the agency will seek an injunction to stop the sale of meat but does not check the milking operation for violations or extend the injunction to that side of the business,” he said. “I would have liked a stronger statement than just that the agency will consider seeking state regulatory enforcement on a case-by-case basis.”

The feds are taking no regulatory action against the dairy farms from the study that had drugs in their milk. In a concession to the dairy industry, the study was “blind,” so the agency cannot trace the tainted samples back to an individual dairy farm or even region of the country.

Unlike pathogens such as salmonella and E. coli, these drug residues cannot be destroyed by cooking. The FDA said it is unlikely that the contaminated milk samples pose a health threat to the consumer since milk from an individual farm is pooled with milk from other farms, diluting it.

The FDA’s findings come amid growing national concern about the impact that overmedicating livestock has on human health. While livestock may be drugged to treat or prevent illness, the animals also have been routinely fed antibiotics to help them gain weight faster. The use of antibiotics – whether in animals or humans – accelerates the growth of antibiotic-resistant bacteria, which can cause illnesses that are more difficult to treat.

Discovered in 1928, antibiotics have been lifesavers: Infections that once killed people have been cured thanks to the drugs. But the more we use them, the more bacteria adapt to survive them. Health experts now warn that our careless use of antibiotics is squandering these miracle drugs and risking the return of life-threatening illnesses.

Each year, at least 2 million people in the U.S. get sick from antibiotic-resistant bacteria, according to the Centers for Disease Control and Prevention. Some 23,000 of them die as a direct result of these infections.

In December 2013, the FDA sought to curb the practice of pumping livestock with antibiotics by making it illegal to use the drugs to make animals grow faster. This week, McDonald’s pledged to phase out serving chicken in its U.S. restaurants that have been raised with antibiotics that also are used on humans.

Katharine Mieszkowski

Katharine Mieszkowski is a senior reporter and producer for Reveal. She's also been a senior writer for Salon and Fast Company. Her work has appeared in The New York Times, Rolling Stone, Mother Jones, Slate and on NPR's "All Things Considered."

Her coverage has won national awards, including the Alfred I. duPont-Columbia University Award two years in a row, an Online News Association Award, a Webby Award and a Society of Environmental Journalists Award. Mieszkowski has a bachelor's degree from Yale University. She is based in Reveal's Emeryville, California, office.

Joanna Lin is a data reporter for Reveal. She previously covered K-12 education and health for California Watch, a project of The Center for Investigative Reporting. She also was a reporter for the Los Angeles Daily Journal and the Los Angeles Times, where she covered breaking news and wrote stories on religion and legal affairs for the metro and national desks. A San Francisco Bay Area native and graduate of the University of Southern California, Lin is based in Reveal's Emeryville, California, office.