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

Calling it “an impossible dilemma,” Doctors Without Borders explained in detail last month’s decision in response to a New York Times story on the case. It would have been the first time the experimental drug was tried in humans.

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

Ebola has killed more than 1,000 people and sickened nearly 2,000 in the current West African outbreak that has also hit Guinea, Liberia and Nigeria. Many of the dead are health workers, who are often working with inadequate supplies and protection.

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

The experimental drug, ZMapp, is designed to boost the immune system to help it fight the virus. Since Khan’s body was already producing an immune response, the doctors may have feared that any boost would kick it into overdrive.

In the end, the treating physicians decided against using the drug. They never told Khan of its existence because they felt it would be unethical to tell him of a treatment they might not use. Shortly after their decision, however, Khan’s condition worsened, the statement said, and the company providing the medical evacuation decided not to transfer him. He died a few days later, on July 29.

“Every day, doctors have to make choices, sometimes difficult, about treatment for their patients,” said the Doctors Without Borders statement. “Trying an untested drug on patients is a very difficult decision, particularly in the light of the ‘do no harm’ principle.”

ZMapp has since been given to two U.S. aid workers and a Spanish missionary priest. The Americans are improving, although it is unclear what role ZMapp has played in that, but the priest died Tuesday.

The last known doses of ZMapp arrived in Liberia on Wednesday, carried personally by Foreign Minister Augustine Ngafuan. The California-based company that makes the drug, Mapp Pharmaceuticals, has said that its supplies are exhausted.