Neither trial was placebo controlled, generally considered the gold standard for clinical data.
In a randomized, controlled trial of 150 patients with mild to moderate COVID-19 in China, researchers found that patients on hydroxychloroquine did not get better significantly faster than those not treated with the drug. Adverse events were also higher in patients receiving the malaria drug.
An observational study of patients in France found that the drug did not significantly reduce admission to intensive care or death in patients hospitalized with COVID-19 pneumonia who required oxygen. COVID-19 is the disease caused by the novel coronavirus.
Demand for hydroxychloroquine surged after Trump touted it in early April, and U.S. regulators have since authorized its emergency use for coronavirus patients.
But the drug has not been proven effective against the disease. Moreover, the U.S. Food and Drug Administration has warned against the use of hydroxychloroquine in COVID-19 patients outside of the hospital or clinical trials due to the risk of serious heart rhythm problems.
The drug is still being widely studied in the United States and abroad as a potential COVID-19 treatment.
The National Institutes of Health (NIH) said on Thursday it began a study to evaluate the combination of antibiotic azithromycin and hydroxychloroquine, which Trump described as a potential "game changer" for the pandemic.
The mid-stage study, for which Teva Pharmaceutical Industries Ltd will be donating medicines, will assess whether the combination can prevent hospitalization and death from COVID-19.
The National Institute of Allergy and Infectious Diseases (NIAID), part of the NIH, is sponsoring the trial, which is being conducted by the NIAID-funded AIDS Clinical Trials Group (ACTG).
"Although there is anecdotal evidence that hydroxychloroquine and azithromycin may benefit people with COVID-19, we need solid data from a large randomized, controlled clinical trial to determine whether this experimental treatment is safe and can improve clinical outcomes," NIAID Director Anthony Fauci said on Thursday.
The NIH's latest study will enroll about 2,000 adults at clinical sites across the country, with many of those expected to be 60 years of age or older or have another condition or chronic disease associated with developing serious complications from COVID-19, such as a cardiovascular disease or diabetes.