US regulators have revoked emergency authorisation for malaria drugs promoted by Donald Trump, for treating coronavirus.
It follows growing evidence that hydroxychloroquine and chloroquine do not work and could cause serious side effects.
The US Food and Drug Administration (FDA) said the drugs are unlikely to be effective in treating Covid-19.
Citing reports of heart complications, the FDA said the drugs' unproven benefits "do not outweigh the known and potential risks".
The US currently has the highest coronavirus death toll worldwide, with more than 116,000 fatalities recorded.
The decades-old drugs, also prescribed for lupus and rheumatoid arthritis, can cause heart rhythm problems, severely low blood pressure and muscle or nerve damage.
The move means that shipments of the drugs obtained by the federal government will no longer be distributed to state and local health authorities for use against coronavirus.
The drugs are still available for other uses, so doctors in the US could still prescribe them for Covid-19 - a practice known as off-label prescribing.
Steven Nissen, a Cleveland Clinic researcher who has been a frequent FDA adviser, agreed with the decision and said he would not have granted emergency access in the first place.
"There has never been any high-quality evidence suggesting that hyrdoxychloroquine is effective" he said, but there is evidence of serious side effects.
On Thursday, a National Institutes of Health panel of experts revised its recommendations to specifically recommend against the drug's use - except in formal studies.
The actions by the FDA and the NIH send a clear signal to health professionals against prescribing the drugs in the treatment of coronavirus.
The president aggressively pushed the drug in the first weeks of the outbreak and stunned medical professionals when he revealed he took the drug pre-emptively against infection.
No large, rigorous studies have found the drugs safe or effective for preventing or treating Covid-19, while a string of recent studies made clear they could do more harm than good.
The FDA granted emergency use of the drugs for Covid-19 patients in late March - at the same time the US government accepted 30 million doses of hydroxychloroquine and chloroquine that had been donated by two foreign drug manufacturers.
Millions of those doses were shipped to US hospitals to treat patient who were not enrolled in clinical trials.
But the FDA previously warned doctors that it had seen reports of dangerous side effects and heart problems reported to poison control centres and other health systems.
The agency said on Monday that it revoked the authorisation in consultation with the Biomedical Advanced Research and Development Authority, which had requested the emergency use.
The authority's former director said in April that he had been removed from his job because he resisted political pressure from Trump appointees to allow widespread use of the malaria drugs.
Rick Bright said he worked with FDA senior staff to limit the drugs’ authorisation to patients in hospital with Covid-19 and under professional supervision.
Among other issues, he objected to the fact that some of the doses imported by the US government were manufactured at facilities in India and Pakistan that had not been reviewed by the FDA.
The FDA said it sampled and tested the imported drugs to confirm they met the agency's standards for safety and quality.