The Liberian man being treated for Ebola virus at a Dallas hospital is being given an experimental drug, according to Texas Health Presbyterian in Dallas. It's an emergency step authorized by the Food and Drug Administration.

Thomas Eric Duncan's family in North Carolina told NBC News that doctors asked them to approve the use of an experimental drug to treat him. The family agreed and said he has already been given his first dose. Texas Health Presbyterian said the first dose was given to Duncan on Saturday afternoon.

Duncan is receiving the investigational medication, brincidofovir, an oral antiviral drug being tested to fight more common viruses, including one that infects patients undergoing bone marrow transplants, Texas Health Presbyterian said. Laboratory tests suggest it might also fight Ebola.

Doctors had sought federal permission to use the drug, which is in late-stage testing for other types of viruses, according to the drug's maker Chimerix Inc. The company did not identify the physicians making the request.

The FDA grants emergency access to unapproved drugs on a case-by-case basis, usually when a patient faces a life-threatening condition for which there are no alternatives. The agency has not approved any drugs or vaccines to safely and effectively treat Ebola.

Two other experimental drugs developed specifically for Ebola have been used to treat American patients, though it's unclear whether they had any effect.

The small supply of one drug, ZMapp, was exhausted after being used on a few patients. A second drug, TKM-Ebola from Tekmira Pharmaceuticals, has been used in at least one patient and is said to be in limited supply.

The drug used to treat Duncan was tested on his blood in a Petri dish and seemed effective, his family said.

Duncan arrived in Dallas on Sept. 20 and fell ill a few days later. After an initial visit to the emergency room at Texas Health Presbyterian Hospital, he was sent home, even though he told a nurse he had been in disease-ravaged West Africa. He returned to the hospital two days later, on Sept. 28, and has been kept in isolation ever since.

Duncan was in critical but stable condition Monday, Texas Health Presbyterian Dallas said, after his condition deteriorated over the weekend.

His family said he's on a respirator and kidney dialysis machine.