(BBC NEWS) Specialists at the Royal Free Hospital in Hampstead, where the nurse is being treated, did not name the drug.
Dr Michael Jacobs said Ms Cafferkey was in isolation and was sitting up in bed, talking and reading.
As the disease has no known cure and is unpredictable, Dr Jacobs said they would know more in a week’s time.
He said Ms Cafferkey had agreed to all the treatments and her family had been to see her.
“She’s a nurse, a fellow professional, so we have been able to discuss things in great detail,” he said.
“She’s as well as we can hope for at this stage of the illness. She’s had the treatment, it’s gone very smoothly, no side-effects at all.”
The drug and the convalescent plasma are part of a tranche of experimental treatments, he said.
“We simply don’t know what the best treatment strategies are,” he added.
Analysis – By James Gallagher, Health editor, BBC News website
This is a difficult time for doctors. Potential Ebola drugs exist yet there is hardly a scrap of evidence that any of them work.
Two of the most promising are brincidofovir and favipiravir.
Tests in tissue samples suggest the anti-virals have the potential to stop Ebola replicating once it infects cells.
Brincidofovir has been tried on some patients in the US and both drugs have entered clinical trials in West Africa.
However, the results are not expected until February.
The effectiveness of using the blood of survivors is equally uncertain until the results of trials come through.
Ebola therapies and vaccines are now being tested at unprecedented speed during this outbreak.
But for patients infected today, doctors simply do not know if these experimental options will make a difference. (read more)