The researchers were unable to confirm why the drug combination caused blood sugar to rise.
But they estimated that in 2009, a total of 715,000 Americans were given the two drugs together.
For those who already had diabetes, the effects of combining the two drugs on blood sugar could require them to take higher doses of drugs to control the condition.
How to avoid diabetes
Diet is the mainstay of diabetes treatment, and research states that dietary advice given to diabetics is equally relevant for prevention.