"Several countries have advocated use of chloroquine and hydroxychloroquine, either alone or in combination, as potential treatments for Covid-19", said Frank Ruschitzka, director of the Heart Centre at University Hospital Zurich and co-author of the study.
The drugs are approved to prevent and treat malaria, and hydroxycloroquine has been approved to treat auto-immune diseases. Trump's endorsement has led many people to take the medications without scientific proof of their benefit. He said he was taking the drug with the approval of the White House physician.
Additional studies have cast doubts on how effective the drug might be in treating the novel coronavirus.
The trial is open to any adults employed at United Kingdom healthcare facilities and working with proven or suspected coronavirus patients, under the condition they have not been diagnosed with COVID-19 or a respiratory disease.
"Instead, our findings suggest it may be associated with an increased risk of serious heart problems and increased risk of death", he said in a journal news release.
A new study published Friday says COVID-19 patients who were treated using chloroquine or hydroxychloroquine - a drug touted by President Donald Trump as a cure for the coronavirus - were more likely to die than those who did not take the drug.
The report in the journal Lancet is not a rigorous test of hydroxychloroquine or chloroquine, but it is by far the largest look at their use in real world settings, spanning 671 hospitals on six continents.
The patients were admitted to 671 hospitals between December 20 and April 14, and all had either been discharged or had died by April 21.
"If drugs as well tolerated as chloroquine and hydroxychloroquine could reduce the chances of catching COVID-19 this would be incredibly valuable..."
But until the results of those trials emerge, chloroquine and hydroxychloroquine should not be used to treat COVID-19 patients, the researchers said. "What have you got to lose?" he asked.
Those who were treated with hydroxychloroquine had a mortality rate of 18 percent. And over 81,000 patients were in the control group.
The highest rate (8%) was among patients given hydroxychloroquine and an antibiotic, compared with 0.3% of patients in the control group.
Doctors have cautioned that the drug carries the risk of serious side-effects on the heart's functions.
"It really does give us some degree of confidence that we are unlikely to see major benefits from these drugs in the treatment of COVID-19 and possibly harm", said Aronoff, who was not involved in the research.
Authors of a separate study that supported the use of antimalaria drugs with antibiotics for COVID requested that their paper be withdrawn, according to the Retraction Watch website.
Prior studies have concluded much the same.
Those patients who received the drugs had a death rate roughly twice as high as those who did not receive the drugs, with the rate significantly higher for those that also received the macrolide in addition to hydroxychloroquine or chloroquine.
Soon after, the U.S. Food and Drug Administration issued a statement that "hydroxychloroquine and chloroquine have not been shown to be safe and effective for treating or preventing COVID-19".
The Indian Council of Medical Research had earlier recommended this drug for healthcare workers at Covid-19 hospitals and the household contacts of coronavirus patients.