Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

WASHINGTON — As the U.S. scales up purchase and use of the drug hydroxychloroquine to treat coronavirus patients, a leading Mayo Clinic cardiologist is sounding a warning: Anyone promoting the drug also needs to flag its rare but serious — and potentially fatal — side effects.

President Donald Trump has repeatedly touted the potential benefits of hydroxychloroquine, which has been approved by the Food and Drug Administration to treat malaria, lupus and other autoimmune ailments but hasn't yet been proven effective and safe in treating the coronavirus.

Full coverage of the coronavirus outbreak

"What do you have to lose?" Trump asked Saturday at the White House when pressed by reporters about hydroxychloroquine's effectiveness. And while he's suggested that patients consult with their physicians about the treatment, he's also said the drug can "help them, but it's not going to hurt them."

On Tuesday, when asked about the drug’s potential side effects, he downplayed them. “The side effects are the least of it,” said Trump. “You’re not gonna die from this pill,” he said. “I say ‘try it’” he said, noting “I’m not a doctor” and to get a physician’s approval.

But the president's reassurance is raising concerns among experts about the dangers the drug poses to some.

After observing the debate over hydroxychloroquine on TV news and in social media, Dr. Michael Ackerman, a genetic cardiologist who is director of the Mayo Clinic's Windland Smith Rice Genetic Heart Rhythm Clinic, took the unusual step in late March of issuing guidance for physicians.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

"What disturbed me the most was when I was seeing not political officials say these medications are safe but seeing on the news cardiologists and infectious disease specialists say" hydroxychloroquine "is completely safe without even mentioning this rare side effect," Ackerman said in an interview.

"That's inexcusable," he added.

Ackerman and his Mayo Clinic colleagues created a cardiac algorithm, published in Mayo Clinic Proceedings, to help physicians more safely prescribe hydroxychloroquine by identifying patients at greatest risk for drug-induced sudden cardiac death.

While hydroxychloroquine is likely to be safe for 90 percent of the population, Ackerman said, it could pose serious and potentially lethal risks to a small number of those susceptible to heart conditions, especially those with other chronic medical problems already on multiple medications.

In fact, a small recent study showed that up to 11 percent of coronavirus patients on hydroxychloroquine and azithromycin are in the so-called "red zone" for potential cardiac side effects.

"They've entered the danger zone," Ackerman said. "That is not just my hunch that patients are going to be reacting to this drug — but they are reacting to this drug."

Ackerman wouldn't directly comment on whether Trump should be promoting the drug from the presidential podium. But he said he was alarmed that the conversation had gotten "way too nonmedical and nonscientific."

He added: "This particular medicine is probably the only medication I know of that has become a Republican or a Democrat medication. That's just crazy."

Prescribing physicians must assess individual patient risk by establishing the so-called QTc interval — an indicator of the health of the heart's electrical recharging system, Ackerman said.

A patient with a significantly prolonged QTc is at increased risk for severe disturbances of the heart's electrical system that can lead to sudden cardiac death.

Download the NBC News app for full coverage and alerts about the coronavirus outbreak

Ackerman said there are as many as 100 FDA-approved medications that can potentially prolong the QTc interval like hydroxychloroquine, and one of them is azithromycin. This combination, which Ackerman refers to as the “corona cocktail,” could be particularly dangerous in the wrong patient.

"It just means that the perfect storm is brewing. It's sort of like walking to the edge of the cliff. You are getting closer and closer to that proverbial edge," Ackerman said.

Hydroxychloroquine's fate as an FDA-approved drug to treat the coronavirus remains uncertain, Ackerman added.

"Yes, these medications overall are really, really safe, so in that sense the president is right. But really safe in a population sense doesn't mean that drug is going to be safe enough for the particular patient I'm about to treat," he said.