The U.S. Food and Drug Administration on Friday cautioned against the use of malaria drug hydroxychloroquine, touted by President Donald Trump, in COVID-19 patients outside of hospitals and clinical trials, citing risks of serious heart rhythm problems.

The decades-old drug has been called a "game changer" by Trump in the fight against the novel coronavirus and anecdotal reports that it may provide some benefit have spurred sales of the drug.

Within hours of Trump's initial endorsement on March 19, pharmaceutical supply chain experts reported shortages as doctors began prescribing hydroxychloroquine for themselves and their families. Patients have also pressured doctors to use the therapies widely touted by Trump and other supporters.

The FDA said on Friday it was aware of increased use of hydroxychloroquine and chloroquine through outpatient prescriptions and the drugs could cause abnormal heart rhythms and dangerously rapid heart rate.

The agency's announcement comes a day after the European Union's drug regulator warned of the side effects of the drugs, urging medical professionals to closely monitor patients on the medicines.