The drug US President Donald Trump said he was taking to ward off Covid-19 actually increases the risk of patients with the disease dying from it, a study in the Lancet has found.
The study said there were no benefits to treating patients with the anti-malarial drug hydroxychloroquine.
Mr Trump said he was taking the drug despite public health officials warning that it could cause heart problems.
The president has repeatedly promoted the drug, against medical advice.
Hydroxychloroquine is safe for malaria, and conditions like lupus or arthritis, but no clinical trials have recommended the use of hydroxychloroquine for coronavirus.
The Lancet study involved 96,000 coronavirus patients, nearly 15,000 of whom were given hydroxychloroquine – or a related form chloroquine – either alone or with an antibiotic.
The study found that the patients were more likely to die in hospital and develop heart rhythm complications than other Covid patients in a comparison group.
The death rates of the treated groups were: hydroxychloroquine 18%; c