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%; chloroquine 16.4%; control group 9%. Those treated with hydroxychloroquine or chloroquine in combination with antibiotics had an even higher death rate.
The researchers warned that hydroxychloroquine should not be used outside of clinical trials.
Mr Trump says he has not tested positive for Covid-19 and is taking the drug because he thinks it has "positive benefits".
A trial is under way to see whether the anti-malarial drug could prevent Covid-19. More than 40,000 healthcare workers from Europe, Africa, Asia and South America who are in contact with patients with the disease will be given the drug as part of the trial.
When asked about the Lancet study, White House coronavirus taskforce co-ordinator Dr Deborah Birx said the US Food and Drug Administration had been "very clear" about concerns in using the drug as either a coronavirus prevention or as a treatment course.
Dr Marcos Espinal, director of the Pan American Health Organization - part of the World Health Organization - has stressed that no clinical trials have recommended the use of hydroxychloroquine for coronavirus.