President Donald Trump touted a two-drug cocktail Saturday to battle the coronavirus. But the medicines together have been linked to cardiac arrhythmia and sudden death.

Concerned physicians responded to Trump’s tweet crowing about what could be “one of the biggest game-changers in the history of medicine” with warnings about the serious side effects — and urgent pleas not to try to obtain the drugs without a medical prescription in consultation with a doctor.

HYDROXYCHLOROQUINE & AZITHROMYCIN, taken together, have a real chance to be one of the biggest game changers in the history of medicine. The FDA has moved mountains - Thank You! Hopefully they will BOTH (H works better with A, International Journal of Antimicrobial Agents)..... — Donald J. Trump (@realDonaldTrump) March 21, 2020

Hydroxychloroquine, sold under the brand name Plaquenil, and the antibiotic Azithromycin in combination have shown some early promise against COVID-19. But the drugs together have not been approved as safe or effective against the disease by the federal Food and Drug Administration. Any use must take into consideration a potentially deadly side effect.

Trump referred to the drugs at his Saturday press briefing and asked: “What do we have to lose? I feel very good about it.”

After the president left the briefing, his top science adviser, Dr. Anthony Fauci of the National Institute of Allergy and Infectious Diseases, was asked if he had “any sense” of the drug cocktail the president referred to, and whether Trump’s “74-and-a-half million Twitter followers should be taking medical advice from him or from someone else.”

Fauci responded that there may be positive “anecdotal reports” — informally observed individual cases — about the drugs, but they haven’t yet been proven to work, or to be safe. “If you really want to definitively know if something works, you got to do the kind of trial that [gives you] the good information,” he said. “The president is talking about hope for people,” not science, he noted.

Please don't take hydroxychloroquine (Plaquenil) plus Azithromycin for #COVID19 UNLESS your doctor prescribes it. Both drugs affect the QT interval of your heart and can lead to arrhythmias and sudden death, especially if you are taking other meds or have a heart condition. — Dr. Edsel Salvana (@EdselSalvana) March 21, 2020

🛑Hydroxychloroquine (plaquenil) can prolong the QT interval (heart rhythm) and can cause a FATAL arrhythmia. Azithromycin can also prolong the QT interval, so use of these 2 drugs together should ONLY be done under the supervision of a doctor. pic.twitter.com/ghtYMmFjBA — Dr. Dena Grayson (@DrDenaGrayson) March 21, 2020

The combination of hydroxychloroquine and azithromycin to treat the coronavirus has not been proven safe and effective through large scale clinical trials. There is only anecdotal evidence from case reports in countries overseas. Promising them as miracle drugs gives false hope. — Eugene Gu, MD (@eugenegu) March 21, 2020

Taking Hydroxychloroquine & Azithromycin together causes a drug interaction that increases risk of irregular heartbeat, which may be life-threatening. No one should take these drugs together without consulting a doctor.https://t.co/r5tQ2cFOiDhttps://t.co/ielNQBIIQl https://t.co/S4jy4gbpIg — Ted Lieu (@tedlieu) March 21, 2020

Infectious disease expert Dr. Matt McCarthy, a physician at Manhattan’s New York-Presbyterian Hospital treating people suffering from COVID-19, tweeted Friday about the drug combination. He noted that a new study (treating 20 patients) showed the drugs decreased the presence of the coronavirus in the blood of those tested. But he also warned about the possibility of fatal arrhythmia. “We don’t really know” if the drugs will cure or kill someone, he tweeted.

Both plaquenil & azithromycin can potentially cause a cardiac disturbance called prolonged QTc. Taken together, these drugs might cause a fatal arrhythmia (torsades de pointes). Or they might help clear virus from the blood. We don't really know. Strong opinions on both sides. — Matt McCarthy (@DrMattMcCarthy) March 20, 2020

Trump at Thursday’s briefing first touted the anti-malarial drug chloroquine as having “tremendous promise” against the coronavirus. He falsely suggested it had been approved for use against COVID-19. “It’s been approved ... they took it down from many, many months to immediate,” Trump said.

But FDA Commissioner Stephen Hahn immediately contradicted Trump, telling reporters: “That’s a drug that the president has directed us to take a closer look at as to whether an expanded use approach. ... We want to do that in the setting of a clinical trial, a large pragmatic clinical trial.”

Trump responded that he had a “good feeling” about the drug. “It’s been around for a long time, so we know that if things don’t go as planned it’s not going to kill anybody,” Trump said.

Officials in Nigeria reported two chloroquine poisonings that resulted in hospitalizations after Trump pushed the drug, Bloomberg noted.

“Please don’t panic,” a senior health official warned Nigerians via a text message, Bloomberg reported Saturday. “Chloroquine is still in a testing phase in combination with other medication and not yet verified as a preventive, treatment or curative option.”

#FactsNotFear@WHO has NOT approved the use of chloroquine for #COVID19 management. Scientists are working hard to confirm the safety of several drugs for this disease.



Please DO NOT engage in self-medication. This will cause harm and can lead to death.#COVID19Nigeria pic.twitter.com/K6kljq0VtW — NCDC (@NCDCgov) March 20, 2020

A study by the Wuhan Institute of Virology found that people could die with just twice the recommended daily dosage of chloroquine, Bloomberg reported.

China tried the drug as a treatment for the coronavirus after early positive clinical trials, but within days officials warned health care professionals of its lethal side effects, according to Bloomberg.