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The US Food and Drug Administration (FDA) has issued an emergency use authorization for chloroquine and hydroxychloroquine to treat patients hospitalized with Covid-19.

The drugs -- which are used to treat malaria and other conditions -- have been called game changers by President Donald Trump.

But thus far, there is little scientific evidence that chloroquine, or its closely-related analogue hydroxychloroquine, are effective in treating Covid-19.

What happened? The authorization came in a letter dated Saturday, but the US Department of Health and Human Services (HHS) acknowledged the FDA’s action in a Sunday news release. The FDA limited the scope of its authorization to drugs supplied from the Strategic National Stockpile. The HHS announced that two pharmaceutical companies -- Bayer and a division of Novartis -- had donated the drugs to the stockpile.

Do the drugs work? In its statement, HHS said:

“Anecdotal reports suggest that these drugs may offer some benefit in the treatment of hospitalized COVID-19 patients.

"The safety profile of these drugs has only been studied for FDA approved indications, not COVID-19."

While there's limited evidence on the efficacy of chloroquine, or hydroxychloroquine, the FDA said the drugs’ benefits outweighed their risk. In its letter, the FDA encouraged randomized clinical trials that could assess the effectiveness of the drugs. It also noted that the known and potential benefits outweigh the risks.

Who can the drugs be used to treat? The authorization is limited to patients who are currently hospitalized and weigh at least 50kg, or about 110 pounds. Under the emergency use authorization, health care providers must contact their local or state health department to access the drugs.