"Hydroxychloroquine and chloroquine can cause abnormal heart rhythms such as QT interval prolongation and a dangerously rapid heart rate called ventricular tachycardia," the agency wrote in the notice. "We will continue to investigate risks associated with the use of hydroxychloroquine and chloroquine for COVID-19 and communicate publicly when we have more information."

The agency also said it became aware of reports of "serious heart rhythm problems" in patients with the virus who were treated with the malaria drugs, often in combination with antibiotic azithromycin, commonly known as a Z-Pak. It also warned physicians against prescribing the drugs to treat the coronavirus outside of a hospital.

The agency said patients taking the drugs for approved reasons, including malaria or to treat autoimmune conditions like lupus, should continue taking their medicine as prescribed.

The Food and Drug Administration warned consumers Friday against taking malaria drugs chloroquine and hydroxychloroquine to treat Covid-19 outside a hospital or formal clinical trial setting after deaths and poisonings were reported.

An employee checks the production of chloroquine phosphate, resumed after a 15-year break, in a pharmaceutical company in Nantong city in east China's Jiangsu province Thursday, Feb. 27, 2020.

The peer-reviewed Journal of the American Medical Association released a new study Friday that said researchers cut short a study testing chloroquine as a potential treatment for Covid-19 over safety concerns, citing a "primary outcome" of death. Nearly two dozen patients died and some developed irregular heart rates after taking doses of the drug daily.

The FDA's warning was issued after JAMA shared the study results with the scientific community but before they were released to the public on Friday.

There are no proven treatments for Covid-19, which has infected more than 2.7 million people worldwide and killed at least 191,231 as of Friday morning, according to data compiled by Johns Hopkins University. U.S. health officials say a vaccine to prevent the disease will take at least 12 to 18 months, meaning finding an effective drug treatment soon is crucial.

Chloroquine and hydroxychloroquine are known to have serious side effects, including muscle weakness and heart arrhythmia.

Chloroquine was approved by FDA in 1949 to treat malaria. Its derivative, hydroxychloroquine, is often used by doctors to treat rheumatoid arthritis and lupus. The drugs have been touted by President Donald Trump as a potential "game changer" in the fight against the coronavirus.

Both drugs are in clinical trials examining their effectiveness in treating the Covid-19, but neither are proven treatments.

Chloroquine gained a lot of attention after a small study of 36 Covid-19 patients published March 17 in France found that most patients taking the drug cleared the coronavirus from their system a lot faster than the control group. Adding azithromycin to the mix "was significantly more efficient for virus elimination," the researchers said. Infectious disease experts and scientists warned that the findings were not definitive.

The FDA has cleared some emergency use of the drugs, but the agency said Friday they have not been approved to prevent or treat the coronavirus.