Why does niacin increase blood sugar?

Insulin resistance. Niacin causes Insulin resistance. In most people, your body simply makes more Insulin to overcome this, and there is no change in blood glucose. In someone who is diabetic, or prediabetic, or obese, or with a family history of diabetes, this worsening of Insulin resistance can push you into diabetes, or make your blood sugar worse. Unfortunate, since the lipid effects are very good.