New York/Los Angeles — The decades-old drug that President Donald Trump has persistently promoted as a potential weapon against Covid-19 has within a matter of weeks become a standard of care in areas of the US hit hard by the pandemic — though doctors prescribing it have no idea whether it works.

Doctors and pharmacists from more than half a dozen large health-care systems in New York, Louisiana, Massachusetts, Ohio, Washington and California say they are routinely using hydroxychloroquine on patients admitted to hospital with Covid-19...

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