U.S. regulators revoked emergency-use authorization for two malaria drugs touted by President Donald Trump as COVID-19 treatments after determining they were unlikely to work against the virus and could have dangerous side effects.

Chloroquine and hydroxychloroquine “are unlikely to be effective in treating COVID-19,” the Food and Drug Administration said Monday.

“Additionally, in light of ongoing serious cardiac adverse events and other serious side effects, the known and potential benefits of CQ and HCQ no longer outweigh the known and potential risks for the authorized use,” the agency said.

The decision is the latest twist in a saga that has created confusion in the race to find therapies for COVID-19. The use of hydroxychloroquine to treat coronavirus patients was embraced early on by hospitals, and a rush to secure supplies led to shortages, but use has dwindled as other treatments gained favor. Trump’s enthusiasm for the medicines also made them a political lightning rod.

The FDA had authorized the drugs in March for patients hospitalized with COVID-19 after Trump called them game changers despite a lack of clinical evidence. Trump said that he took hydroxychloroquine himself to try to ward off possible infection.

The FDA attempted Monday to depoliticize its decision.

“We’ve made clear throughout the public health emergency that our actions will be guided by science and that our decisions may evolve as we learn more about the SARS-CoV-2 virus, review the latest data, and consider the balance of risks versus benefits of treatments for COVID-19,” FDA Deputy Commissioner for Medical and Scientific Affairs Anand Shah said in a statement.

The request for emergency authorization had come from an agency within the Department of Health and Human Services called the Biomedical Advanced Research and Development Authority. On Monday, the unit requested it be withdrawn, according to the FDA’s letter to acting BARDA head Gary Disbrow.

Clinical-trial results “led BARDA to conclude that this drug may not be effective to treat COVID-19 [Coronavirus Disease 2019] and that the drug’s potential benefits for such use do not outweigh its known and potential risks,” according to the letter.

Former BARDA Director Rick Bright had clashed with administration officials over the promotion of hydroxychloroquine as a coronavirus treatment. Bright said in a complaint filed with the Office of Special Counsel last month that he was ousted from the agency over his concerns regarding the drugs.

The FDA had been waiting on randomized, controlled clinical trials on the drugs -- the most reliable means of assessing a treatment -- as it weighed whether to maintain the emergency authorization in the face of observational analyses that concluded the drugs didn’t work and could be harmful.

“Recent data from a large randomized controlled trial showed no evidence of benefit for mortality or other outcomes such as hospital length of stay or need for mechanical ventilation of HCQ treatment in hospitalized patients with COVID-19,” the FDA said in the letter.

Hydroxychloroquine has been hotly debated since the early days of the pandemic. Since then, doubts about its efficacy and safety have been raised by both studies and regulators. In France, health authorities said heart risks and deaths had been reported.

Late last month, a global study of patient records in the Lancet medical journal suggested similar dangers, but the article was retracted after doubts rose about the data underlying the study. Since then, a robust study from researchers at the University of Oxford failed to show benefits from using the drugs in COVID patients.

The FDA cited the Oxford study in materials that accompany its letter to BARDA saying its “results offer persuasive evidence of a lack of benefit of HCQ in the treatment of hospitalized patients with COVID-19.”

The FDA also said 109 cases of serious adverse cardiovascular events in patients taking the drugs were reported to its online database, including 25 deaths. Of the cases, 75 per cent reported fast, chaotic heartbeats, a condition known as QT prolongation and most patients were taking a drug, including the antibiotic azithromycin, known for causing such a condition.

Hydroxychloroquine is the most-studied treatment for COVID-19 with more than 200 clinical trials listed on a National Institutes of Health registry that are researching its use against the virus. NIH is conducting its own study on whether the drug can prevent hospitalization and death. The agency didn’t immediately respond to a request for comment.

Hydroxychloroquine was used much more often than chloroquine on COVID-19 patients. Prescriptions for the drug shot up after Trump began to push the drugs in mid-March but had fallen over recent weeks as analyses of the medications weren’t positive.

Hydroxychloroquine usage in U.S. hospitals peaked in late March, with more than half of COVID-19 patients receiving the generic malaria drug. As of June 8, only four per cent of hospitalized patients with the disease have been given the drug, according to CarePort Health, which collects hospital drug utilization from electronic health record companies across the country.

Hydroxychloroquine and chloroquine are approved to treat malaria, and hydroxychloroquine is also FDA cleared to treat lupus and rheumatoid arthritis.

--With assistance from John Lauerman and Riley Griffin.