BACKGROUND Recurrent aphthous stomatitis (RAS) is a painful condition of unknown etiology, affecting more than 2.5 billion people worldwide. Vitamin deficiencies have been implicated as a possible cause. METHODS The authors conducted a single-center, randomized, parallel-arm, double-masked, placebo-controlled study to examine the effect of daily multivitamin supplementation on the number and duration of RAS episodes. The authors randomly assigned 160 adults who had a validated history of at least three episodes of idiopathic minor RAS within the previous 12 months to one of two groups: the first group (n = 83) received a once-daily multivitamin containing 100 percent of the U.S. reference daily intake (RDI) of essential vitamins, and the second group (n = 77) received once-daily placebo for up to 365 days. RESULTS The results showed no significant difference in the mean number of new RAS episodes between the multivitamin (4.19 episodes) and placebo (4.60 episodes) arms during the study period (P = .69). The mean duration of new RAS episodes also was similar for the multivitamin (8.66 days) and placebo (8.99 days) arms (P = .60). Furthermore, the authors found no differences between the two arms with regard to mouth pain, normalcy of diet or compliance with the study medication regimen. CONCLUSIONS Daily multivitamin supplementation, with the RDI of essential vitamins, did not result in a reduction in the number or duration of RAS episodes. CONCLUSIONS Clinicians should not recommend multi-vitamin supplementation routinely as prophylaxis for RAS.