For the first time, a drug is showing promising signs of effectiveness in Ebola patients participating in a study. The medicine, which interferes with the virus’s ability to copy itself, seems to have halved mortality — to 15 percent, from 30 percent — in patients with low to moderate levels of Ebola in their blood, researchers have found. It had no effect in patients with more virus in their blood, who are more likely to die.

The drug, approved as an influenza treatment in Japan last year, was generally well tolerated.

“The results are encouraging in a certain phase of the disease,” Dr. Sakoba Keita, director of disease control for the Guinean Ministry of Health, said in a telephone interview. The drug is being tested in Guinea, one of the three West African countries most affected by the Ebola crisis.

The details of the early findings have not yet been announced, but they raise questions about which patients, if any, outside the study should be offered treatment with the drug, favipiravir. “These are very difficult, agonizing decisions,” said Susan Ellenberg, a professor of biostatistics at the University of Pennsylvania’s Perelman School of Medicine, who was not involved in the research. She cautioned that early results were sometimes not borne out.

The drug has been provided on an emergency basis to Ebola patients in European countries, but not in Africa. The Japanese maker of the drug announced in October that it had 20,000 courses of treatment in stock. The epidemic is now ebbing but is not over. The World Health Organization on Wednesday reported 124 new cases in Guinea, Sierra Leone and Liberia in the week that ended on Sunday, warning of an increased geographical spread in Guinea and a rise in new cases in all three countries for the first time this year.