The drug Cymbalta (duloxetine), one of two drugs approved for treating the pain of diabetic peripheral neuropathy, appears to adversely impact control of blood sugar, according to researchers reporting in the journal Diabetes Care.

Dr. Thomas Hardy from Eli Lilly and colleagues pooled data from three clinical trials and found short-term treatment with Cymbalta linked to a modest increase in fasting sugar levels but not to significant increases in hemoglobin A1c, a measure of long-term sugar control.

But the researchers found that when patients were treated with duloxetine for a longer period, there was a significant rise in hemoglobin A1c, indicating worsened sugar control.

Duloxetine does not treat the underlying nerve damage of peripheral neuropathy, but is thought to reduce the pain by enhancing the nerve signals within the central nervous sytem that naturally inhibit pain.

"We hope these findings reported here will help (doctors) make informed decisions regarding the risks and benefits of duloxetine in patients seeking relief from painful diabetic neuropathy," the researchers concluded.