ObjectiveTo assess the effect of vitamin D and calcium supplementation in preventing recurrences of benign paroxysmal positional vertigo (BPPV).MethodsWe performed an investigator-initiated, blinded-outcome-assessor, parallel, multicenter, randomized controlled trial in 8 hospitals between December 2013 and May 2017. Patients with confirmed BPPV were randomly assigned to the intervention (n=518) or the observation (n=532) group after a successful treatment with canalith repositioning maneuvers. The primary outcome was the annual recurrence rate (ARR). Patients in the intervention group had taken vitamin D 400 IU and 500 mg of calcium carbonate twice a day for one year when serum vitamin D level was lower than 20 ng/ml. Patients in the observation group were assigned to follow-ups without further vitamin D evaluation or supplementation.ResultsThe intervention group showed a reduction in the ARR [0.83 (95% CI: 0.74-0.92) vs. 1.10 (95% CI: 1.00-1.19) recurrences per one-person year] with an incidence rate ratio of 0.76 (95% CI: 0.66-0.87, p<0.001) and an absolute rate ratio of -0.27 (-0.40 to -0.14) from intention to treat analysis. The number needed to treat was 3.70 (95% CI: 2.50-7.14). The proportion of patients with recurrence was also lower in the intervention than in the observation group (37.8 vs. 46.7%, p=0.005).ConclusionsSupplementation of vitamin D and calcium may be considered in patients with frequent attacks of BPPV, especially when serum vitamin D is subnormal.Classification of Evidence:This study provides Class III evidence that for patients with BPPV, vitamin D and calcium supplementation reduces recurrences of BPPV.