Brazilian researchers stopped a study testing an old malaria drug chloroquine for the treatment of COVID-19, the disease caused by the new coronavirus, as some patients developed cardiac complications after taking higher doses of the drug.

The researchers found that in one group of patients that took high dose chloroquine, some of them developed potentially dangerous heart rhythm problems.

Sold under the brand name Aralen, Chloroquine and its close cousin hydroxychloroquine (Plaquenil) have been making headlines in recent times after US President Donald Trump described the drugs as a potential “game-changer” for treating COVID-19.

The Brazilian study planned to enroll 440 patients to determine whether chloroquine is a safe and effective drug for the treatment of COVID-19. However, the researchers were able to enroll just 81 patients.

Some were given a high dose of chloroquine (600 mg) twice a day for 10 days, while some received a low dose (450 mg) for 5 days, with a double dose on the first day.

As the study progressed, the researchers were concerned after seeing a few warning signs. Within a few days, some patients in the high-dose group developed heart rhythm problems, and two of the patients in that group experienced a fast, abnormal heartbeat called ventricular tachycardia before dying.

The researchers immediately stopped the high-dose group of the study after the findings and cautioned against using high doses of chloroquine for any patient with COVID-19.

The authors wrote in their paper, “Our study raises enough red flags to stop the use of such [high] dosage … worldwide in order to avoid more unnecessary deaths.”

Similarly, a hospital in France has stopped treating a COVID-19 patient with hydroxychloroquine after the patient experienced arrhythmia, per the Newsweek.

Chloroquine and hydroxychloroquine are often used for the treatment and prevention of malaria and are generally well-tolerated. However, one major complication of these drugs is the risk of developing a serious heart condition known as “QT prolongation,” according to the researchers.

Live Science previously reported that these antimalarial drugs are unsafe for patients who already have heart rhythm problems.

The Brazilian researchers stopped the high-dose group and reverted all patients from that group to the low-dose group.

All the participants were also given antibiotic azithromycin, which is also known to increase the risk of arrhythmia problems.

Some hospitals in the United States are trying a combination of hydroxychloroquine and azithromycin to treat COVID-19 patients, according to The New York Times. The article is originally published in Live Science.