A possible treatment for COVID-19, heavily touted by U.S. President Donald Trump in recent weeks, has been withdrawn from high-dose use in a Brazilian trial because it was feared to be having adverse effects on patients’ hearts.

Chloroquine, normally a drug used to treat malaria, has inspired hope in some medical circles amid reports that it can help to offset the symptoms of the deadly virus. But the New York Times reports that a recent clinical trial saw patients develop irregular heartbeats, a number of whom developed heart arrhythmias and later died.

The similar drugs chloroquine and hydroxychloroquine have been pushed by Trump as a possible partial solution to the pandemic in the U.S., where they are sold under brand names like Nivaquine and Plaquenil. The Times reports that the U.S. Food and Drug Administration has granted emergency approval for the use of both drugs at U.S. hospitals, and U.S. companies have begun producing them in greater numbers. Trump has been vociferous on the drugs’ potential benefits, but among those to contradict his views are his own top expert Anthony Fauci, who has provided regular updates to Americans on the crisis. During one recent daily White House coronavirus briefing, Trump stepped in and prevented Fauci from answering a question about hydroxychloroquine.

Experts say that in reality there is not much to back up theories that chloroquine and hydroxychloroquine are effective against COVID-19. Cholroquine was first touted as a treatment during the MERS outbreak of 2012, Science Alert reports, but it wasn’t pursued because it wasn’t proving to be effective.

The Brazil study was undertaken with 81 patients at a hospital in Manaus, in Amazonas state. As well as chloroquine, patients were also given azithromycin, an antibiotic also being used in the U.S., often paired with chloroquine. However, higher dose use of chloroquine (600 milligrams as opposed to 450 milligrams) was stopped after just six days, after 11 of patients who were given the greater dose died.

The news will likely fuel further doubts about the efficacy of the drugs; in recent days, statements advocating caution around their use have not been hard to find.

Robert A. Harrington of the American Heart Association, said in a recent statement on azithromycin and hydroxychloroquine:

“The urgency of COVID-19 must not diminish the scientific rigor with which we approach COVID-19 treatment. While these medications may work against COVID-19 individually or in combination, we recommend caution with these medications for patients with existing cardiovascular disease.”

Dr. David Juurlink, head of the division of clinical pharmacology at the University of Toronto, told the New York Times of the Brazil study:

“To me, this study conveys one useful piece of information, which is that chloroquine causes a dose-dependent increase in an abnormality in the ECG (electrocardiogram) that could predispose people to sudden cardiac death.”

At least one province in China, Guangdong, had recommended similarly high doses of chloroquine to treat patients, the Times reports, and Dr. Marcus Lacerda, who worked on the Brazil study, told the Times that the new work showed that “the high dosage that the Chinese were using is very toxic and kills more patients.”

“That is the reason this arm of the study was halted early,” he said.

One New York area hospital told the Times it does not plan to stop using the hydroxychloroquine-azithromycin combination in lower doses, on patients who aren’t severely ill.

On April 7, the U.S. Centers for Disease Control and Prevention removed from its website highly unusual guidance informing doctors on how to prescribe hydroxychloroquine and chloroquine.

The move came three days after Reuters reported that the CDC published key dosing information involving the two antimalarial drugs based on unattributed anecdotes rather than peer-reviewed science.

— with files from Reuters