The FDA made clear Friday that the drug has not been shown to be "safe and effective for treating or preventing Covid-19," and carries "known risks" of potentially deadly heart complications. The FDA said those risks "may be mitigated when health care professionals closely screen and supervise these patients such as in a hospital setting or a clinical trial," which is mentioned in an emergency-use authorization for the drugs issued last month.

"While clinical trials are ongoing to determine the safety and effectiveness of these drugs for Covid-19," the FDA said on Friday, "there are known side effects of these medications that should be considered."

The FDA's warning comes just days after CNN previously reported on a study conducted at VA medical centers that showed coronavirus patients taking hydroxychloroquine were no less likely to need mechanical ventilation and had higher deaths rates compared to those who did not take the drug. That study was based the treatment of hundreds of patients at US Veterans Health Administration medical centers.

But when asked whether -- in the wake of the FDA's warning and the study done on veterans -- the VA believes its patients should use hydroxychloroquine going forward, spokeswoman Christina Noel said the VA is using "it to treat Covid-19 in cases where Veteran patients and their providers determine it is medically necessary, and in a manner consistent with current FDA guidance."

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