For several weeks US Presidents Donald Trump has been pushing malaria drug hydroxychloroquine as a possible way of treating coronavirus.
However, a study in US veterans hospitals found that not only was there no benefit, but there were more deaths among those given the drug compared to those who received standard care.
Researchers analysed the medical records of 368 male patients admitted to hospital with confirmed coronavirus at Veterans Health Administration medical centres who died or were discharged by April 11.
And they discovered about 28 per cent of those who were given hydroxychloroquine plus usual care died, compared to 11 per cent of those who received routine care alone.
Scientists in Brazil recently stopped part of a study testing chloroquine, an older drug similar to hydroxychloroquine, after heart rhythm problems developed in a quarter of patients.
On Tuesday, NIH issued new treatment guidelines from a panel of experts, saying there was not enough evidence to recommend for or against chloroquine or hydroxychloroquine for COVID-19.
But the potential side effects were highlighted.