Treating latent tuberculosis normally requires nine months of daily pill-taking, but a new study shows that a far shorter course of medication, with once-a-week drugs, works just as well.

The study will greatly simplify the lives of about 300,000 Americans who undergo the treatment each year, and the new regimen may reduce the number of cases of active tuberculosis.

“This is the biggest breakthrough in the treatment of latent TB since the 1960s,” said Kevin Fenton, head of tuberculosis prevention at the Centers for Disease Control and Prevention.

Tuberculosis is caused by a slow-growing bacterium that usually infects the lungs. In most cases the body’s immune system controls the organism, so it doesn’t cause disease. Such “latent” cases can turn into organ-destroying infections, however, if a person’s immune defenses are weakened by AIDS, cancer, old age or poor nutrition.

To prevent that from happening, doctors advise preventive treatment for people exposed to someone with active tuberculosis and for those whose test results indicate a recent latent infection. For decades, that has meant taking a daily dose of the drug isoniazid for nine months.

The new study compared that regimen to a simpler one — a higher dose of isoniazid taken once a week, and another drug, rifapentine, also taken weekly, both for three months.

The study, of about 8,000 volunteers in the United States, Canada, Brazil and Spain over three years, found seven cases of active TB among those taking the short course and 15 among those taking the traditional one.

The short course is more expensive — $503 vs. $237 — but more people completed it (82 percent) than the longer one (69 percent). About 4 percent of people in each group had side effects severe enough to stop the treatment.

About 2 billion people have latent TB infection, and about 1.7 million people die of active TB each year. In the United States, about 11 million people are latently infected, and last year 11,181 developed active tuberculosis.

The new strategy has been studied less extensively in places with high TB incidence, and it is uncertain at this point whether it will be used there, said Kenneth Castro, also of CDC’s tuberculosis department.

The study, which took 10 years to complete, was funded by CDC and led by Timothy Sterling, a physician at Vanderbilt University.