A Study of Seasonal Variation in the Effect of Add-On Sitagliptin on Blood Glucose Control in Insulin-Treated Patients With Type 2 Diabetes.

Background: There are several reports of seasonal variation in hemoglobin A1c (HbA1c) in patients with type 2 diabetes (T2DM), but no reports of seasonal variation in the effect of add-on drugs on blood glucose control in insulin-treated patients.

Methods: Using data collected from 630 patients in a multicenter study, we compared the amount of change in HbA1c after 1, 3, 6, 9, and 12 months of add-on administration of sitagliptin in insulin-treated patients divided into four groups based on the month when sitagliptin was started.

Results: A significantly larger decrease in HbA1c at 6 months from baseline was observed in the group that started add-on sitagliptin in February to April than in the other three groups. However, the amount of change in HbA1c at 12 months did not differ among the groups.

Conclusions: The consideration of seasonal variation enables more accurate evaluation of a drug's short-term effect on blood glucose control.