The malaria drug hydroxychloroquine -- which President Trump said he took in the hope of warding off Covid-19 -- was found not to prevent infections among volunteers in a study released on Wednesday.

The study, which was ended early, included 125 health care workers -- some of whom took hydroxychloroquine daily for eight weeks while the others took a placebo.

"There was no significant difference in infection rates in participants randomized to receive hydroxychloroquine compared with placebo," the researchers wrote in the study, published in the journal JAMA Internal Medicine.

Based on their findings, the researchers from the University of Pennsylvania wrote that they "cannot recommend the routine use of hydroxychloroquine" among health care workers to prevent Covid-19.

Participants enrolled in the new study from April to July. It found that four of the 64 healthcare workers who were randomly given hydroxychloroquine ended up testing positive for Covid-19 and four of the 61 healthcare workers who were given a placebo tested positive.

Among those eight participants who tested positive, six developed symptoms, none required hospitalization and they all clinically recovered from the illness, according to the study.

Overall, the findings in the new research are similar to what was reported in another study in the New England Journal of Medicine back in June. That previous study found that hydroxychloroquine did not prevent illness when used within four days of being exposed to the coronavirus that causes Covid-19.

In June, Dr. David Boulware, an author of that previous study, told CNN that the President's physician sent him an email on May 9, seeking his opinion about taking the drug preventatively, and asking about the results of the study and the dose the study subjects were taking.

Boulware said he advised Trump's physician that there was no published research showing hydroxychloroquine worked preventatively and shared that the people in his study who took hydroxychloroquine had higher rates of side effects, mostly gastrointestinal problems such as nausea and vomiting.

"I knew they were probably going to ignore what I said because the White House had been talking about hydroxychloroquine for weeks and weeks and weeks," said Boulware, an infectious disease expert and professor of medicine at the University of Minnesota.

"Even in the setting of a pandemic, we need research to help inform the best practice for what works in humans," Boulware said.

In July, the US Food and Drug Administration revoked an emergency use authorization for hydroxychloroquine and chloroquine to treat Covid-19. The agency now says hydroxychloroquine and chloroquine have not been shown to be safe and effective for treating or preventing Covid19.

CNN's Elizabeth Cohen contributed to this report.

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