An Arizona man has died and his wife is in a critical condition after both ingested chloroquine phosphate in an apparent effort to stave off Covid-19.
President Donald Trump has touted the medication form of chloroquine as a possible treatment for the virus.
But the toxic ingredient taken by the Phoenix couple was a chemical commonly used to clean fish tanks.
Shortly after ingestion, the couple fell ill, said Arizona-based hospital system Banner Health.
The couple were both in their 60s.
The woman told NBC News she had seen a televised briefing in which Mr Trump discussed the potential benefits of chloroquine as a treatment Covid-19.
The drug has been approved by the US Food and Drug Administration (FDA) for treating malaria, lupus and rheumatoid arthritis but is unproven for use against the novel coronavirus.
"We saw his press conference. It was on a lot, actually," she said. "Trump kept saying it was basically pretty much a cure."
"We were afraid of getting sick," she told NBC.
There are at least 46,548 confirmed cases of the virus throughout the US, with at least 592 deaths.
The couple had used chloroquine previously as a treatment for their koi fish and still had some remaining in their home. This additive is formulated differently than the drug used to treat malaria.
They mixed a small amount of the substance with liquid to drink. Within 20 minutes, both fell ill.
"I started vomiting," she said. "My husband started developing respiratory problems."
Her husband could not be revived in hospital and she remains in critical care.
"Banner Health experts emphasise that chloroquine, a malaria medication, should not be ingested to treat or prevent [Covid-19]," the health group said in a statement.
Given the uncertainty around Covid-19, we understand that people are trying to find new ways to prevent or treat this virus, but self-medicating is not the way to do so."
Last week, Mr Trump trumpeted the drug as a potential treatment, writing on Twitter that it had "a real chance to be one of the biggest game changers in the history of medicine," when taken with the antibiotic azithromycin.
"We're going to be able to make that drug available almost immediately," he said last week at a White House coronavirus press briefing.
"And that's where the FDA has been so great. They've gone through the approval process - it's been approved."
The FDA soon clarified that chloroquine had not been approved, saying there are currently no therapeutics or drugs approved by the body to "treat, cure or prevent Covid-19".
On Sunday, Dr Anthony Fauci - the nation's top infectious disease expert - said on CBS News that the president had heard about the two drugs from anecdotal reports.
"I… have said I'm not disagreeing with the fact anecdotally they might work, but my job is to prove definitively from a scientific standpoint that they do work," he said.
Mr Fauci, who serves on the White House coronavirus task force, is widely seen as the face of the US response.
Last week, Nigerian health officials also issued a warning regarding the drug's misuse after hospitals in Lagos, the nation's capital, began reporting patients suspected of chloroquine poisoning.
At least three people are believed to be hospitalised and reports have spread of high demand of chloroquine leading to pharmacy shortages, partially fuelled by Mr Trump's comments.
The World Health Organisation "has NOT approved the use of chloroquine for #Covid19 management," the Nigeria Centre for Disease Control said in a statement on Friday.
"Please DO NOT engage in self-medication. This will cause harm and can lead to death."