There is no evidence the drug works as a prophylactic for COVID-19, for which two White House staffers tested positive about a week and a half ago. It was around that time, Trump said, that he started taking hydroxychloroquine, after consulting the White House physician; he did not say exactly prompted him to do so, but he denied it was because he had been "exposed" to the virus, as a reporter put it.
“I don’t have any information about the exact rationale," White House press secretary Kayleigh McEnany said in an interview with Fox News Tuesday morning, calling Trump's decision the result of "a personal deliberation" between the president and his physician.
McEnany said Trump was trying to be "transparent about his personal health decision," but his admission came around after a rambling response to an unrelated question from a reporter, which itself came after a lengthy meeting he held with restaurant executives.
Trump had spoken to members of the press a number of times since he said he started using the medication without mentioning it; likewise, before Monday the White House had made no mention of him taking it.
While the president’s statement came as a surprise, he has previously suggested the drug may have preventative qualities in protecting against contracting the virus in the first place and even said in early April that he may be personally inclined to take it.
“I may take it. And I’ll have to ask my doctors about that, but I may take it," the president said during an April 4 briefing during which he spoke about a “rumor out there” that patients who were being treated for lupus with hyrdoxychloriquine had not been contracting the coronavirus.
Dr. Anthony Fauci, the nation’s top infectious disease expert, said at the time that “we don’t have any definitive information to be able to make any comment” on the drug’s prophylactic qualities.
While several large, observational studies have shown no benefit of taking hydroxychloroquine to COVID-19 patients, Trump said Monday he had heard only one "negative" about it -- one observational study of hundreds of patients in veterans hospitals across the country. (An observational study looks at the effects of a treatment without intervention.)
The study, published last month without first going through the typical scientific peer-review process, looked at the drug's use in early stages of treatment and found a higher rate of death among patients who used hydroxychloroquine compared to those who received standard care.
Trump said "people that aren't big Trump fans gave it," providing no evidence to back that claim or what the significance of that was.
On Tuesday, he went further, calling the study “a Trump enemy statement” – again without providing any evidence or reasoning for his assertion, beyond saying the patients involved were "very old, almost dead."
The Food and Drug Administration, among other health authorities, have warned against using hydroxychloroquine to treat COVID-19, alone or in combination with azithromycin.
"Hydroxychloroquine and chloroquine have not been shown to be safe and effective for treating or preventing COVID-19," the agency said on April 24, warning using them could cause an abnormal hearth rhythm that could lead to sudden cardiac death.
Risks "may be mitigated when health care professionals closely screen and supervise these patients such as in a hospital setting or a clinical trial," the FDA said.
Trump ignored the wide scientific consensus that there is, in fact, a potential risk to taking it, when he repeatedly praised the drug again on Thursday afternoon, saying "it doesn't hurt people."
"People are going to have to make up their own mind," he said, speaking to reporters on Capitol Hill.
Medical experts have said Trump’s embrace of an unproven treatment – let alone using it as a prophylactic, also unproven – could prove dangerous.
“There is no need to weigh risks and benefits of hydroxychloroquine use for prevention of COVID-19 because there is no consistently demonstrated benefit” and there are well known health risks, Matthew Heinz, a physician in Tucson, Ariz., who served in the Obama administration, said. Heinz assisted with the U.S. response to Ebola while serving in the U.S. Department of Health and Human Services.
Much as was feared when Trump first started pushing the drug, increased demand could lead to shortages for patients who use it to treat malaria, lupus or rheumatoid arthritis.
Despite the lack of scientific evidence, the president’s decision to take the drug amounted to a presidential endorsement of the drug’s ability to fend off the virus.
Even so, his press secretary, McEnany, cautioned against the use of the drug without medical consultation, saying on Tuesday that “any use of hydroxychloroquine has to be in consultation with your doctor, you have to have a prescription.”
While the president is taking the drug under a prescription from the White House doctor, according to a senior official, he said he began taking it not as at the recommendation of his doctor, but at his own request.
“A White House doctor -- didn't recommend -- no, I asked him, ‘What do you think?’ He said, ‘Well, if you'd like it.’ I said, ‘Yeah, I'd like it. I'd like to take it,’” Trump said Monday.
The White House on Monday night released a memorandum from the president’s physician, Sean Conley, that raised more questions than it answered.
“After numerous discussions [Trump] and I had regarding the evidence for and against the use of hydroxychloroquine, we concluded the potential benefit from treatment outweighed the relative risks,” Conley wrote.
But Conley did not say whether he had actually prescribed the drug to Trump, whether as a prophylactic or otherwise, and he did not even confirm that Trump was taking it. McEnany told reporters Tuesday morning Trump was, indeed, taking it as he had said the day before.
Conley did note that Trump “receives regular COVID-19 testing, all negative to date.”
Lacking scientific evidence to support his belief in the drug’s effectiveness, Trump on Monday cited “a lot of positive calls” and a letter he received from a New York doctor who reported anecdotal positive results in using the drug on his patients.
The president suggested he would like to make that letter public, but the White House has so far not done so. One senior official on Tuesday suggested it was unlikely to be released, referring to the letter a private correspondence.
The White House has also declined to offer further details about the president’s prescription, the date he began taking the drug, the dosage and frequency, and whether his heart is being regularly monitored to ensure the president is not having a potentially dangerous reaction to the drug.
ABC News' Libby Cathey contributed to this report.