A long-used AIDS drug appears to be the first effective, long-term treatment for hepatitis B in those with advanced liver disease caused by the virus.

The drug lamivudine, also known as 3TC, has been available for the treatment of hepatitis B since 1998, but the consequences of using it for years in those with serious liver disease or cirrhosis were unknown.

Researchers tested the drug for almost three years in 651 people, mostly Asians, and found it cut in half the risk of liver failure and the chances that the disease would develop into liver cancer.

About 8 percent of those who got lamivudine saw their liver disease get worse, compared with 18 percent of those who were given a dummy pill. The study was ended early because of the difference in the groups, and everyone was offered lamivudine.

The research is reported in Thursday’s New England Journal of Medicine.

“For years and years and years, we had absolutely nothing to offer patients who had advanced hepatitis B-related liver disease. So it’s a significant study,” said Dr. Jack R. Wands of the Liver Research Center and Brown Medical School in Providence, R.I.

Wands said other drugs are in development that could give doctors even more potent options for treating the incurable infection.

About 1.25 million Americans have chronic hepatitis B infections, and 15 percent to 25 percent will die of chronic liver disease, according to government estimates. The hepatitis B virus is transmitted through blood, bodily fluids, and shared needles and from mother to child. Hepatitis B infections have been declining in the United States because of childhood vaccinations.

The study, led by Dr. Yun-Fan Liaw of Chang Gung Memorial Hospital and University in Taipei Taiwan, was conducted in a number of countries in the Asia and Pacific region, where hepatitis B is an even bigger problem.

Liaw said doctors have been hesitant to use lamivudine long-term because some patients become resistant to the drug, which happened to about half of those in the study. If resistance develops, Liaw said, patients can now be switched to the newest hepatitis B drug, adefovir dipivoxil, or Hepsera. Hepsera was approved two years ago; its long-term effects are not yet known.

A third hepatitis B drug, interferon, has side effects and is generally used in patients with less advanced liver disease.

The study was paid for by GlaxoSmithKline, which sells lamivudine as Epivir-HBV, and company employees were involved in the research. Some of the study’s researchers have received speaking fees from GlaxoSmithKline or support from other drug makers. Wands has received a research grant from a drug company developing another hepatitis B drug.