A man in Arizona has died after ingesting chloroquine phosphate - believing it would protect him from contracting coronavirus.
The man's wife also ingested the substance and is receiving critical care, NBC News reported.
She said they had watched US President Donald Trump speaking about the potential benefits of chloroquine treating coronavirus during televised briefings.
However, the toxic ingredient the couple consumed was not the medication form of chloroquine used to treat malaria in humans, but the ingredient listed on a parasite treatment for fish.
While no drugs are approved to prevent or treat COVID-19, the disease caused by the coronavirus, some early research suggests chloroquine could aid it.
The man's wife, who asked to remain anonymous, said she had previously used chloroquine to treat her koi fish.
She told NBC News: "I saw it sitting on the back shelf and thought , 'Hey, isn't that the stuff they're talking about on TV?'"
The couple, both in their 60s and potentially at higher risk for complications from the virus, mixed a small amount of the substance with liquid and drank it.
"We were afraid of getting sick," she said.
Both became extremely ill within 20 minutes, at first feeling "dizzy and hot".
"I started vomiting," the woman said.
"My husband started developing respiratory problems and wanted to hold my hand."
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Donald Trump said there had been 'very encouraging results' in tests of chloroquine to tackle COVID-19
She called 911 and when paramedics arrived, they "were asking a lot of questions" about what the couple had consumed.
"I was having a hard time talking, falling down," she said.
Her husband died shortly after arriving at hospital.
There are currently 41,242 confirmed coronavirus cases in the US and 462 deaths.
While the worst outbreaks are concentrated in certain parts of the US, such as New York, health experts warn the highly infectious disease is certain to spread.
Mr Trump said last week that two old anti-malaria drugs, hydroxychloroquine and chloroquine, could help fight coronavirus.
US fast-tracks approval for virus treatments
The president said chloroquine, which is also used for severe arthritis, lupus and autoimmune condition Sjogren's syndrome, has shown "very encouraging early results" and is set to be approved for coronavirus use quickly.
"We're going make that drug available almost immediately... it could be available in days," he said.
"It's a medical war, we have to win this war."
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On Monday, Mr Trump told reporters he wants to reopen the US for business in weeks, not months, as he claimed continued closures could result in more deaths than the virus itself.
"We have to open our country because that causes problems that, in my opinion, could be far bigger problems," he said.