LONDON (AP) — The European Union’s medicines regulator on Thursday warned countries that malaria drugs being used experimentally to treat the new coronavirus have potentially serious side effects, including seizures and heart problems.

The European Medicines Agency said in a statement that chloroquine and hydroxychloroquine — two medicines embraced by U.S. President Donald Trump and others as a potential COVID-19 treatment — are known to cause heart rhythm problems, especially if combined with other drugs.

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There is currently no licensed treatment for COVID-19 and dozens of trials are under way globally. Chloroquine and hydroxychloroquine have long been used to treat malaria and anti-inflammatory diseases like rheumatoid arthritis. In addition to the heart problems, the two drugs can also cause side effects including liver and kidney damage, seizures, and result in low blood sugar.

“Clinical data are still very limited and inconclusive, and the beneficial effects of these medicines in COVID-19 have not been demonstrated,” the EMA said. It noted that several clinical trials testing the drugs’ effectiveness against the coronavirus are using higher than recommended doses, which it said could increase the risk of side effects including abnormal electric activity that could disrupt the heart rhythm.

Earlier this month, part of a study in Brazil was suspended after doctors found one quarter of patients taking chloroquine developed irregular heart rhythms after taking a higher dose.

In the U.S., doctors reported disappointing results this week for a study investigating the use of hydroxychloroquine for COVID-19. Among 368 patients who took the drug with or without the antibiotic azithromycin, there were more deaths among those who took the drug versus those who received standard care.

The EMA reminded doctors and patients to report any suspected side effects from hydroxychloroquine and chloroquine to national authorities. “These must not be used without a prescription and without supervision from a doctor,” the agency said.

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