This is a prospective, randomized, double-blind, placebo-controlled 8-week clinical trial to evaluate the efficacy of niacinamide versus placebo to reduce plasma phosphorus levels in peritoneal dialysis patients. Patients on a stable regimen of phosphate binders with plasma phosphorus levels > 4.9 mg/dL are eligible for enrollment and randomization. The study will span 8 weeks of active medication treatment. The primary end-point will be absolute change in plasma phosphorus levels. Sixteen patients (8 randomized to each treatment arm) are needed to detect a 1.5 mg/dL difference assuming a standard deviation of 1 and powered at 80% (alpha 0.05). Placebo will be packaged to resemble the study drug in all physical attributes. The starting dose of niacinamide will be 250 mg twice daily to be titrated to 500 mg twice daily at the end of week 2 with a final titration to 750 mg twice daily at the end of week 4. Secondary end-points will include absolute change in the calcium-phosphorus product, intact parathyroid hormone, hemoglobin, platelet count, total cholesterol, HDL cholesterol, and percentage change in plasma phosphorus.