An anti-malaria drug that President Donald Trump has persistently promoted as a potential weapon against COVID-19 has quickly become a standard way of treating patients in parts of the U.S. hit hard by the pandemic.

But doctors prescribing it tell Reuters they have no idea whether it works.

Last month Trump tweeted that malaria drug hydroxychloroquine and an antibiotic called azithromycin could be "one of the biggest game changers in the history of medicine." Two days later he repeated this optimism at a news conference.

(SOUNDBITE) (English) U.S. PRESIDENT DONALD TRUMP, SAYING (MARCH 23, 2020): "The hydroxychloroquine and the Z-pack, it's looking very, very good and it's going to be distributed." Since his statements, doctors and pharmacists from more than half a dozen large healthcare systems in New York, Louisiana, Massachusetts, Ohio, Washington and California told Reuters they are routinely using hydroxychloroquine on patients hospitalized with COVID-19.

But they don't know whether patients who improve after prescription are cured by the drugs, or would have resolved the disease on their own.

The regional director of critical care at Northwell Health, a 23-hospital system in New York, said in an email to Reuters QUOTE, "I have seen hundreds of patients with severe COVID and most of these people are on hydroxychloroquine... In my opinion, although it is very early, I do not see a dramatic improvement from the hydroxychloroquine in these patients." Hydroxychlorquine's effectiveness remains unproven.

The top U.S. infectious diseases expert said as much when Trump first began promoting the treatment last month.

(SOUNDBITE) (English) DIRECTOR OF THE NATIONAL INSTITUTE OF ALLERGY AND INFECTIOUS DISEASES, DR. ANTHONY FAUCI, SAYING (MARCH 21, 2020): "I'm not totally sure what the president was referring to, but I believe he's referring to a report that used both hydroxychloroquine and azithromycin together to have some possibility of being in effect.

Many of the things that you hear out there are what I had called anecdotal reports.

They may be true, but they're anecdotal." Currently there are no drugs approved to treat the novel coronavirus, and doctors report facing pressure from patients demanding the drug that they've heard endorsed by the president.

The treatment isn't without risk: Potential side effects of hydroxychloroquine include vision loss and heart problems. But doctors interviewed by Reuters say they are comfortable prescribing the drug for a short course of several days for coronavirus patients because the risks are relatively low and the therapies are inexpensive and generally available.