LONDON (Reuters) – British scientists halted a major drug trial on Friday after it found that the anti-malarial hydroxychloroquine, touted by U.S. President Donald Trump as a potential “game changer” in the pandemic, was “useless” at treating COVID-19 patients.

“This is not a treatment for COVID-19. It doesn’t work,” Martin Landray, an Oxford University professor who is co-leading the RECOVERY trial, told reporters.

“This result should change medical practice worldwide. We can now stop using a drug that is useless.”

Vocal support from Trump raised expectations for the decades-old drug that experts said could have been a cheap and widely available tool, if proven to work, in fighting the pandemic, which has infected more than 6.4 million people and killed nearly 400,000 worldwide

Controversy surrounding the drug grew after a study published in medical journal The Lancet last month raised safety concerns and led several COVID-19 studies of it to be halted. The Lancet study was then retracted on Thursday after its authors said they were unsure about its data.

Landray, a professor of medicine and epidemiology at Oxford University, noted the “huge speculation” about the drug as a treatment for COVID-19 but said there had been until now “an absence of reliable information from large randomised trials”.

He said the preliminary results from RECOVERY, which was a randomised trial, were now quite clear: hydroxychloroquine does not reduce the risk of death among hospitalised patients with COVID-19.

“If you’re admitted to hospital, don’t take hydroxychloroquine,” he said.

The RECOVERY trial of hydroxychloroquine had randomly assigned 1,542 COVID-19 patients to hydroxychloroquine and compared them with 3,132 COVID-19 patients randomly assigned to standard care without the drug.

Results showed no significant difference in death rates after 28 days, in length of stay in hospital or in other outcomes, the researchers said.

The World Health Organization (WHO) said on Wednesday it would resume tests of hydroxychloroquine as part of its ‘Solidarity’ trials, after those running the study briefly stopped giving it to new patients in light of the Lancet paper.

Peter Horby, an Oxford University professor who is co-leading the RECOVERY trial with Landray, said his team had informed the WHO of their decision to halt the UK study.

“There was a call this morning with the WHO to give them a heads-up on the results… They… said they would be convening their committee to look again at their decision regarding the Solidarity trial.”