Several states have urged doctors to stick to prescribing the medicines only for their proven uses or have restricted prescriptions to a certain number of days to avoid hoarding after prescription rates soared across the country; shortages have quickly arisen for patients who have used the product for years to treat lupus and arthritis. Other lawmakers such as Sen. Elizabeth Warren (D-Mass.) have demanded plans from the administration about how to secure supplies for those longtime patients.

Johnson’s letter argues that restricting the drug to hospitalized patients results in the “loss of the critical early window of opportunity” to prevent the spread of the virus and reduce hospitalizations.

But critics of the administration’s plan to dole out hydroxychloroquine for off-label use say that prescribing it outside of hospitals will make it even more difficult to track effectiveness or risks, like heart problems, that can spring up. There is no evidence that the drug helps prevent coronavirus and limited data to suggest it helps people early on in their infection, though trials are ongoing.

Johnson has long sought to loosen FDA restrictions; he helped write "right to try" legislation to expand access to experimental drugs and he cites patients' right to try in this latest campaign.

His office said that so far, the letter has the backing of 776 practicing physicians. He’s already submitted it to the White House but is collecting more signatures until Trump responds.

The letter also pushes back on criticism that hydroxychloroquine is an unproven way to address the coronavirus because no definitive clinical results have come out yet and some early studies suggest no impact at all.

“Evidence of successful treatment in thousands of patients is accumulating from many countries as well as U.S. physicians; it is far beyond ‘anecdotal,’” the letter said.