BACKGROUND: Hyperphosphatemia remains a significant problem for patients requiring dialysis and is associated with increased mortality. Current treatment options include dietary restriction, dialysis, and phosphate binders. Treatment using the latter is frequently limited by cost, tolerability, and calcium loading. One open-label trial found niacinamide to be effective at decreasing serum phosphorus values in hemodialysis patients. Niacinamide may effectively reduce phosphorus levels in peritoneal dialysis (PD) patients already receiving standard phosphorus-lowering therapies. METHODS: An 8 week, randomized, double blind, placebo-controlled trial to evaluate the effectiveness of niacinamide to reduce plasma phosphorus levels in PD patients. Patients had to demonstrate a baseline phosphorus value > 4.9 mg/dL. Patients were randomized to niacinamide or placebo and prescribed 250 mg twice daily, with titration to 750 mg twice daily, as long as safety parameters were not violated. Phosphate binders, active vitamin D, and cinacalcet were kept constant during the study. The primary end point was change in plasma phosphorus. Secondary end points included changes in lipid parameters. RESULTS: 15 patients started on the study drug (8 niacinamide, 7 placebo) and 7 in each arm had at least one on-study phosphorus measurement. The niacinamide treatment group experienced an average 0.7 +/- 0.9 mg/dL decrease in plasma phosphorus and the placebo-treated group experienced an average 0.4 +/- 0.8 mg/dL increase. The treatment effect difference (1.1 mg/dL) was significant (p = 0.037). No significant changes in high- or low-density lipoproteins or triglycerides were demonstrated. Two of the 8 patients randomized to the niacinamide treatment arm had to withdraw from the study due to drug-related adverse effects. Adverse effects may limit the use of niacinamide in PD patients. CONCLUSION: Niacinamide, when added to standard phosphorus-lowering therapies, resulted in a modest yet statistically significant reduction in plasma phosphorus levels at 8 weeks. [ClinicalTrials.gov number NCT00508885].