Brazilian researchers stopped a small study of the potential Covid-19 treatment, Chloroquine, after patients developed fatal heart complications.

The study of 81 patients in the Amazonian city of Manaus, saw half receive a high dose of 600 milligrammes of chloroquine twice daily for 10 days, while the remainder received 450 milligramme doses over five days.

Researchers began noticing heart arrhythmias in patients taking the higher dose after three days, and by the sixth day of treatment 11 patients had died, according to a report on medRxiv, an online pre-publication server for medical articles that have not undergone peer review.

While it was unclear if the deaths were from the coronavirus or the Chloroquine, the study was suspended and the surviving patients placed on the lower dose of chloroquine. The sample size was not large enough to determine the effectiveness of the lower dose.

“Preliminary findings suggest that the higher [chloroquine] dosage (10-day regimen) should not be recommended for COVID-19 treatment because of its potential safety hazards,” researchers wrote. “Such results forced us to prematurely halt patient recruitment to this arm.”

The anti-malaria drugs Chloroquine and Hydroxychloroquine have been given an “Emergency Use Authorization“ by the Food and Drug Administration for treatment of Covid-19.

In the report, researchers said that despite being a safe drug and used for more than 70 years for malaria, Chloroquine might be toxic in the dosages recommended by Chinese authorities.

In addition to Chloroquine, all patients received the antibiotic azithromycin. Researchers were not able to independently assess the toxic role of azithromycin “because all patients were already using this antibiotic as per hospital protocol”.

“In a unique pandemic situation, health professionals have to choose between offering medical assistance and generating and reporting reliable data, a dichotomy that compromises the generation of good quality evidence for clinical management,” the report said.

“Global recommendations for COVID-19 are being made based on unpowered studies, however, and due to the chaotic urgency, such drugs are being prescribed in a compassionate manner given the severity of this disease.

“Our study raises enough red flags to stop the use of such dosage worldwide in order to avoid more unnecessary deaths.”