France reported 43 cases of heart incidents linked to treating coronavirus patients with hydroxychloroquine, the malaria drug President Donald Trump has repeatedly touted as a potential "game changer." 

As the U.S. stockpiles as many as 29 million doses of the drug, which is also used to treat lupus and rheumatoid arthritis, the data on adverse reactions from France's drug safety agency highlights the risk of using unproven treatments to stem a pandemic that's killed more than 100,000 people worldwide.

French President Emmanuel Macron on Thursday visited a clinic in Marseille, southern France, where reports of uncontrolled studies have recently propelled the 65-year-old drug to fame. Macron's advisers said the trip wasn't an endorsement.

France has recorded about 100 health incidents and four deaths linked to experimental drugs for coronavirus patients since March 27, the national drug-safety agency ANSM said in a statement on Friday. Three other patients had to be revived.

Some 82 incidents were deemed "serious." Most of those were split between hydroxychloroquine and HIV antivirals lopinavir-ritonavir, according to the agency.

Didier Raoult, the researcher whose studies have sparked global interest in hydroxychloroquine, has been criticized by the scientific community for his methods -- notably, the lack of a control group to establish a benchmark for the success of his experimental treatments.

Raoult says there's no need to put a group of patients on a placebo to draw conclusions about whether the treatment he's promoting is effective.

Nigeria in March reported cases of chloroquine poisoning after Trump praised the anti-malaria drug as a treatment for the novel coronavirus.

"These drugs should only be used in hospitals, under close medical supervision," France's ANSM said in the statement. The agency cited the "unusual" context and that they're being used for an illness other than those for which they were developed.