The antidepressant Paxil and the statin drug Pravachol can cause a dangerous spike in blood sugar when taken together.

According to a study published in Clinical Pharmacology and Therapeutics, the effect was strong enough to thrust pre-diabetics into full-scale diabetics. The spike was even higher in people who already have diabetes, whose blood sugar is already too high, and could put their health at risk.

After scanning large databases and finding a significant increase in blood sugar levels among people taking the two drugs, researchers tested the theory in mice. When they treated pre-diabetic mice with the drug combo, their blood glucose skyrocketed from 128 mg/dl to 193 mg/dl.

The researchers believe as many as 1 million Americans take the drug combo, and it’s possible that the combination may have precipitated in many of those a diagnosis of Type 2 diabetes.