A randomized, placebo-controlled trial found no evidence hydroxychloroquine protected people exposed to the novel coronavirus that has infected over 1.8 million Americans and counting. On Thursday, the New England Journal of Medicine published results from a post-exposure prophylaxis trial that were more than a little discouraging. This latest disappointment comes four days after Sanofi (NASDAQ:SNY) stopped supplying the drug to COVID-19 patients enrolled in clinical trials. 

No benefit

While hydroxychloroquine can stop SARS-CoV-2 from making copies of itself in a laboratory, it doesn't seem to get the job done for real people. In a study with 821 patients who had been exposed to SARS-CoV-2, 11.8% of those randomized to receive hydroxychloroquine developed symptoms consistent with COVID-19, compared to 14.3% of patients that received a placebo.

Person checking a vial of blood.

Image source: Getty Images.

While there was a numerical trend toward a benefit, the difference wasn't clear enough to rule out random chance. This failure doesn't necessarily mean the old malaria drug is completely useless when it comes to treating COVID-19, but it probably isn't worth the side effects.

At the top of hydroxychloroquine's side effect list is an increased risk of irregular heartbeat that can put someone in the hospital or worse. While there weren't any serious adverse events reported, 40.1% of patients given the malaria drug reported side effects compared to 16.8% of the placebo group.

While the evidence against hydroxychloroquine as a prophylactic or treatment for COVID-19 looks grim, the study did have some limitations. Most notably, asymptomatic patients enrolled without showing consistent proof of exposure and adherence to the prescribed regimen couldn't be monitored.