It’s common knowledge that mixing your medication isn’t recommended without a doctor’s permission. However, an estimated 1 million people in the United States are on two drugs that together can cause a seriously adverse reaction.

This video from Business Insider recalls a study from Harvard, Stanford, and Vanderbilt University on a specific drug interaction. Their study looked at the interaction between pravastatin – a cholesterol-reducing drug – and paroxetine, a widely used antidepressant known under the brand name of Seroxat. If taken together, they can cause complications that are so severe, they could become life-threatening.

Watch the video below to find out why.