Editor's note: On June 4, the study was retracted by three of its authors, after they were unable to complete an independent audit of the data.

Chloroquine and hydroxychloroquine are two of the most hyped drugs being studied as treatments for covid-19, thanks in large part to President Donald Trump’s repeated promotion during his public appearances. Trump told reporters this week he had been taking hydroxychloroquine as a preventive measure. But a new study published Friday in The Lancet suggests not just that the drugs don’t offer any real benefit to infected patients, but that they can increase the risk of heart problems or even death.  

What are these drugs again? Chloroquine and its less toxic alternative, hydroxychloroquine, are widely used antimalarial drugs Since chloroquine was discovered over 85 years ago, it’s been studied pretty extensively. It’s now very cheap to manufacture, and we know its side effects. Some previous research indicates that it can prevent a virus from replicating inside a host cell. We still don’t know exactly how effective these drugs are when it comes to treating covid-19.

The new study: A team of American researchers looked at the records of 14,888 hospitalized covid-19 patients who received one of four treatments: chloroquine alone, chloroquine with a macrolide (a class of antibiotics), hydroxychloroquine alone, or hydroxychloroquine with a macrolide. Those records were compared with those of another 81,144 patients who did not receive any of these drug regimens. 

After controlling for confounding factors (including underlying health conditions), the authors were “unable to confirm a benefit of hydroxychloroquine or chloroquine” when used alone or in one of the other regimens. Moreover, treatment with any of the four drug regimens was actually associated with a higher risk of death and heart ailments. The biggest risk increase was observed in the group treated with hydroxychloroquine and a macrolide—8% of those patients developed a heart arrhythmia, compared with just with 0.3% in the group who received none of the drug treatments. 

Caveats: The study is solely an observational look at previous medical records—it’s not a clinical study that can really prove anything about the drugs’ safety or efficacy. You cannot draw any strong conclusions from it. It's another notch in a larger body of research looking at chloroquine and hydroxychloroquine.