New studies show that treating critically ill COVID-19 patients with inexpensive steroids can cut their risk of dying from the illness by a third. The results are so clear that the World Health Organization changed its advice on Sept. 2 and now strongly recommends corticosteroids as a first-line treatment for the sickest patients.

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Steroids aren’t risk-free, however. They can have side effects, and they could do more harm than good in patients with milder cases of COVID-19.

I am a pulmonologist and critical care physician and co-author of one of three new studies that analyzed data from clinical trials involving the effect of steroids on thousands of critically and severely ill COVID-19 patients. Here’s what people need to understand about steroids as a treatment for COVID-19.