Chicago ? A less common strain of flu has shown hints of resistance to two flu drugs among patients in a small study in Japan, a country known for prescribing the drugs more frequently than anywhere else in the world.

Signs of resistance to the drugs Tamiflu and Relenza turned up among a few patients who had type B influenza, normally a milder flu causing smaller outbreaks than the more common type A.

The findings were troubling to researchers because they suggested doctors will eventually need new medications to treat drug-resistant flu if the viruses become more prevalent.

Previous studies, including work by the same researchers, have found a few cases of resistance to Tamiflu in type A flu, the variety thought most likely to cause a pandemic if bird flu changes into a form that is more easily spread among people, not just poultry.

Dr. William Schaffner, an infectious-disease specialist at Vanderbilt University who was not involved in the study, said Japanese doctors prescribe anti-flu drugs frequently, perhaps too often, giving viruses a chance to evolve. “We were afraid this might happen and, sure enough, it has,” he said. The study underlines the importance of vaccination and other preventive measures, he said.

Some scientists believe Tamiflu and Relenza, which were designed to treat seasonal flu, may also be helpful in treating a global epidemic, although that is not clear.

The U.S. government’s preparation for a flu pandemic includes stockpiling Tamiflu and Relenza, and funding development of new anti-flu drugs, said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.

Anytime doctors treat widely with an anti-viral drug, “you are going to have, sooner or later, the evolution of resistance,” Fauci said. “It’s critical to have a pipeline of drugs you can have available when that resistance develops.”

The new study appears in today’s Journal of the American Medical Association.