Around 1 in 10 people in the US take antidepressants, the most common of which is SSRIs — selective serotonin reuptake inhibitors — which are prescribed for depression, anxiety and panic disorder. These drugs, which include Zoloft and Prozac, can be lifesaving, but they can also stop the body from regulating its temperature properly and decrease heat tolerance.

It’s a worrying side effect doctors and scientists are still trying to understand fully, especially as the world heats up and punishing, prolonged heat waves become more frequent.

SSRIs do enormous good, said Dr. Laurence Wainwright, a lecturer at the University of Oxford who researches the mental health impacts of climate change. They “have been revolutionary in psychiatry,” he told CNN. But, as with any drug, there are a host of side effects. “It’s about getting a cost-benefit ratio right,” he said.