President Trump briefly departed Walter Reed Medical Center on Sunday evening in a SUV to wave to supporters who have gathered outside as he receives treatment after testing positive for the coronavirus. Confusion continued Sunday about the severity of Mr. Trump’s condition, as his physician revealed the president had been given a dose of a powerful steroid that the World Health Organization has recommended for patients who are “critically ill” with COVID-19.

Dr. Sean Conley, the White House physician, said the president had been given dexamethasone, on Saturday, and had experienced two drops in his oxygen levels since the onset of the illness. Dexamethasone has been shown to improve outcomes for patients with severe cases of COVID-19, including those who require supplemental oxygen but is not recommended for use in patients with milder cases.

Conley said the president’s move to Walter Reed on Friday was prompted by Conley’s concern for the “rapid progression” of his symptoms from Thursday night into Friday morning, with a high fever and low oxygen levels. Conley said Mr. Trump was given supplemental oxygen on Friday before he was brought to the medical center.

In a video he tweeted before he departed Walter Reed, Mr. Trump said he had “learned a lot about COVID.”

“I learned it by really going to school,” Mr. Trump said. “This is the real school, this isn’t the ‘let’s read the books school,’ and I get it, I understand it. And it’s a very interesting thing and I’m going to be letting you know about it.”