The Food and Drug Administration said 25 of 26 drug companies that were asked to phase out antibiotics to promote growth in farm animals have agreed to comply with the agency’s voluntary plan.

The development announced Wednesday clears a major hurdle in the FDA’s push to combat growing human resistance to antibiotics because of their overuse.

Farms use about 80 percent of the nation’s antibiotics supply, sometimes in healthy animals to speed up growth or prevent illness in unsanitary conditions.

Their widespread application is being blamed for the rise of superbugs that afflict 2 million people in the U.S. and contribute to 23,000 deaths each year, according to the U.S. Centers for Disease Control and Prevention.

The FDA first proposed a plan in December that asked major drugmakers to remove growth promotion claims on their products, effectively making it illegal to prescribe the drugs to livestock for anything other than medical reasons.

The antibiotics in question are only those with similar applications in human medicine. Known as medically important antimicrobials, they include popular drugs such as penicillin and tetracycline.

Skeptics feared that manufacturers would resist the plan. The FDA said a voluntary approach was necessary because banning the drugs would have resulted in lengthy and costly litigation.

The FDA said the 25 companies that agreed to the plan make 99.6 percent of the targeted drugs. They include major manufacturers such as Bayer Healthcare and Novartis Animal Health.

“The FDA and drugmakers appear to have passed the first big test of the agency’s voluntary approach,” said Laura Rogers, director for the campaign on human health and industrial farming for the Pew Charitable Trusts. “This is very encouraging … but there’s a lot more to do.”