What is dexamethasone and when is it used to treat coronavirus?

The medical team said the president took his first dose of the drug on Saturday and would remain on it for the "time being."

Conley said that Trump's blood oxygen levels dropped to below 94% at one point, which is below the normal range.

But Dr. John Torres, medical corespondent with NBC News, said that the most concerning announcement that came out of Sunday's news conference was that Trump is taking dexamethasone.

"Once they start him on dexamethasone, that means his symptoms are more than mild or moderate," Torres said.

Dexamethasone is recommended by the World Health Organization and the National Institutes of Health for patients seriously ill with coronavirus.

The drug works by reducing inflammation in and around the lungs, which can make it difficult to get enough oxygen into the blood. The treatment is not recommended for mild illness because, in some cases, it can make the infection worse by helping the virus replicate. The NIH recommends against using the steroid for patients who don't require supplemental oxygen.

"The fact that they started him on dexamethasone, to me, is a big, red flag that there is a little bit more going on," Torres said.

Doctors have been worried about using steroids like this to fight a mild and early case of the virus.

"I didn't want to give any information that might steer the course of illness in another direction," he said. "In doing so, it came off that we were trying to hide something, which wasn’t necessarily true. The fact of the matter is, he is doing really well."

The president is considered high risk for the coronavirus because he is 74 years old and clinically obese.

Conley said Trump could be released from Walter Reed National Military Medical Center as early as Monday.