The decision by the National Institutes of Health panel not to recommend either for or against a treatment included the antiviral remdesivir, which is being studied in several trials in the United States and around the world. Data is also lacking about the use of so-called convalescent plasma donated by coronavirus survivors to provide antibodies that might help patients fight the disease.

But the expert panel did specifically advise against several treatments unless they were given in clinical trials. One was the combination of the malaria drug hydroxychloroquine plus the antibiotic azithromycin, which Mr. Trump has repeatedly promoted despite the lack of evidence that they work.

Those drugs should be used only in clinical trials “because of the potential for toxicities,” the experts said.

The panel also had cautionary advice about hydroxychloroquine and the closely related drug chloroquine, even when given without azithromycin, saying that patients receiving them should be monitored for adverse effects, particularly an abnormality in heart rhythm called prolonged QTc interval.

A study of the records of 368 Veterans Affairs patients, posted on Tuesday but not yet peer-reviewed, found that hydroxychloroquine, with or without azithromycin, did not help patients avoid the need for ventilators. And hydroxychloroquine alone was associated with an increased risk of death.

But the study was not a controlled trial, and patients who received the drugs were sicker to begin with. The authors wrote, “These findings highlight the importance of awaiting the results of ongoing prospective, randomized, controlled studies before widespread adoption of these drugs.”

At Tuesday’s White House briefing, Dr. Stephen Hahn, the commissioner of the Food and Drug Administration, described the study as small and retrospective, adding, “what F.D.A. will require is data from randomized clinical trials.”