The World Health Organisation on Monday said it had temporarily suspended clinical trials of hydroxychloroquine as a possible treatment for coronavirus.
The decision came after a study published in The Lancet medical journal last week suggested the drug could increase the risk of death among Covid-19 patients, WHO chief Tedros Adhanom Ghebreyesus said.
Dr Tedros said the executive group of the Solidarity Trial, in which hundreds of hospitals around the world have enrolled patients to test possible treatments for the coronavirus, suspended trials of hydroxychloroquine as a precaution.
"The executive group has implemented a temporary pause of the hydroxychloroquine arm within the Solidarity Trial while the safety data is reviewed by the data safety monitoring board," Dr Tedros said.
"The other arms of the trial are continuing."
Hydroxychloroquine is normally used to treat arthritis but public figures including US President Donald Trump have backed the drug as a coronavirus treatment, prompting governments to buy it in large amounts.
Mr Trump said last week he was taking the drug as a preventive measure, but on Sunday he told Sinclair Broadcasting that he had completed his course.
"Finished, just finished," he said. "And by the way, I'm still here. To the best of my knowledge, here I am."
Brazil's Health Minister last week also recommended using hydroxychloroquine, and the anti-malarial chloroquine, to treat even mild Covid-19 cases.
The Lancet study found both drugs could produce serious side-effects, particularly heart arrhythmia.
Neither drug helped patients in hospital with Covid-19, said the study, which looked at the records of 96,000 patients across hundreds of hospitals.
Dr Tedros said both drugs were generally safe for patients with autoimmune diseases or malaria.
WHO chief scientist Soumya Swaminathan told Monday's briefing that the Solidarity Trial had been looking only at the effects of hydroxychloroquine and not chloroquine.
The decision to suspend enrolment for trials using hydroxychloroquine was "a temporary measure", Dr Swaminathan said.
The Covid-19 pandemic, which began late last year in China, has killed almost 350,000 people worldwide and infected more than 5.5 million.
While there is still no approved treatment or vaccine, drastic measures that at one point saw half of humanity under lockdown have pushed down transmission rates in several countries.
As many nations begin to gradually lift restrictions, the WHO stressed the need to maintain physical distancing and increase screening.
"All countries need to remain on high alert," WHO expert Maria Van Kerkhove said.
"The virus will take the opportunity to amplify if it can."
WHO emergencies chief Michael Ryan warned against the idea that the pandemic might move in seasonal waves.
"Making an assumption that it is on a downward trajectory, and the next danger point is some time in October or November, I think that would be a dangerous assumption," Mr Ryan said.