A relatively common drug combination — the antidepressant Paxil and the cholesterol-lowering Pravachol — may cause rises in blood glucose levels. These findings, published Wednesday, result from a collaboration between investigators at the School of Medicine, Vanderbilt University and Harvard Medical School.

The discovery comes as a surprise since neither of the drugs produces the same blood glucose effect when taken independently. According to researchers, the increase is more severe among diabetics for whom blood glucose control is crucial. The investigators also examined pre-diabetic lab mice examined to both drugs.

Data from the Food and Drug Administration and electronic medical records provided the backbone for the findings.

“These kinds of drug interactions are almost certainly occurring all of the time, but, because they are not part of the approval process by the Food and Drug Administration, we can only learn about them after the drugs are on the market,” said bioengineering professor Russ Altman in a press release.

It is not uncommon for drugs to produce adverse effects when used in combination. But since drugs are tested and approved individually, rendering predictions of combined effects impossible.

Studies of mice showed the same effect when the two medications were used in conjunction; the mice demonstrated a spike in fasting glucose levels when treated with Paxil and Pravachol together for three weeks. But the impact isn’t limited to mice.

“Between 13 and 15 million people in this country have prescriptions for these drugs,” said Altman, who is the study’s senior author. “By extrapolating from the electronic medical records at Stanford and elsewhere, we can predict that between 500,000 and one million people are taking them simultaneously.”

— An Le Nguyen