As the deadliest Ebola outbreak ever unfolds in western Africa, two American missionaries continue to recover in Atlanta after receiving an experimental drug called ZMapp. An Ebola exert explains how ZMapp is derived and how it fights the deadly virus. (Gillian Brockell and Pamela Kirkland/The Washington Post)

Doctors treating a Sierra Leone physician with Ebola defended their decision not to give him an experimental drug, saying Wednesday that they feared it was too risky.

Calling it “an impossible dilemma,” Doctors Without Borders detailed their decision in response to a New York Times report on the case. It would have been the first time the drug was tried in humans.

The explanation came the same day that another top doctor from Sierra Leone died of the disease, intensifying a debate about how to apportion a limited supply of untested drugs and vaccines and whether they are even effective.

The current Ebola outbreak has killed more than 1,000 people and sickened nearly 2,000 in West Africa..

At the time the experimental treatment was being considered for Sheik Humarr Khan, his immune system was already starting to produce antibodies, suggesting that he might recover, Doctors Without Borders said in a statement. Khan was also due to be transferred to a European hospital that would be more capable of handling any problems that might arise, it said.

The statement did not specify what drug was considered. But it is thought to be ZMapp, designed to boost the immune system to help it fight the virus. With Khan’s body already producing an immune response, the doctors might have feared that any boost would kick it into overdrive.

In the end, the treating physicians decided against using the drug. Shortly afterward, however, Khan’s condition worsened, the statement said.. He died a few days later, on July 29.

ZMapp has since been used on two Americans and a Spaniard. The California-based company that makes the drug, Mapp Biopharmaceuticals, has said that its supplies are now exhausted.

The drug has never before been tested in humans, and it is not clear whether it is effective or even harmful. The Americans are improving — although it is unclear what role ZMapp may have played in that — but the Spaniard died Tuesday.

The last known doses of ZMapp arrived Wednesday in Liberia, where they will be given to two sick doctors. They will be the first Africans known to receive the treatment.

The Ebola epidemic over time

Meanwhile, Nigeria confirmed another death from Ebola, bringing the toll in that country to three.