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The Centers for Disease Control and Prevention warned against using chloroquine phosphate to treat coronavirus in official guidance on Saturday, after the agency learned that the substance killed one person and left another critically ill.

"Chloroquine phosphate, when used without a prescription and supervision of a healthcare provider, can cause serious health consequences, including death," the CDC said. "Clinicians and public health officials should discourage the public from misusing non-pharmaceutical chloroquine phosphate (a chemical used in home aquariums)."

The agency added that pharmaceutical chloroquine phosphate and hydroxychloroquine sulfate are approved by the FDA to treat specific medical conditions, such as malaria, lupus, and rheumatoid arthritis. "Currently, these medications are being studied and evaluated as treatment for COVID-19; however, their efficacy to either prevent or treat this infection are unknown."

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NBC News reported on an Arizona couple, both in their 60s, who became deathly ill after they ingested fish food that contained chloroquine phosphate.

The man's wife said she had watched televised briefings during which President Donald Trump talked about the potential benefits of chloroquine.

"We were afraid of getting sick," the wife said.

The couple mixed some of the substance with food and drink, and within 20 minutes both became extremely ill.

"I started vomiting," the woman told NBC News. "My husband started developing respiratory problems and wanted to hold my hand."

She called 911. Shortly after her husband arrived at the hospital, he died.

Studies to find out whether chloroquine would be safe or effective in treating COVID-19 are only just beginning.

The French government announced that it had officially given its permission for the drug to be prescribed to treat COVID-19 on Thursday after seeing some initial promising results in a Marseilles clinic run by renowned biologist Didier Raoult.

President Trump, meanwhile, on Friday defended his promotion of the drug.

We have a pandemic," Trump said at a press conference. "We have people dying now. If we're going to go into labs and test all of this for a long time, we can test it on people right now who are in serious trouble, who are dying. If it works, we've done a great thing. If it doesn't work, you know, we tried."