Recurrent aphthous stomatitis (RAS) is the most common chronic disease of the oral cavity, affecting 5-25% of the population. This condition is characterized by recurrent and painful oral ulcers. The discomfort of RAS can impact negatively on quality of life that is why the therapeutic demand is strong. Currently, there is no curative treatment for RAS, and suspensive treatments such as colchicine and thalidomide are no definitive or can induce severe side effects. The pathophysiology of RAS remains largely unknown. Recent studies suggest that a dysregulation of the oral microbiota may be implicated. Increasing data underline the potential interest of using probiotics in conditions due to microbiota disorder. To the best of our knowledge, no study has evaluated the effectiveness of probiotic in RAS. The main objective of this double blind clinical trial is to evaluate the effectiveness and safety of a probiotic, the Lactobacillus rhamnosus Lcr35®, in the treatment of RAS in adults. 40 patients suffering from minor RAS for 6 months will be included and randomly assigned to 2 group of 20. Patients of group A will took the Lactobacillus rhamnosus Lcr35® orally daily for 3 months and patients of group B will took the placebo similarly. All patients will be then followed up for 3 additional months without treatment. The primary outcome measure will be the monthly number of aphthae occurring during the 3 months of treatment. The secondary outcomes measures will be the monthly number of aphthae occurring during the 3 months of follow up, the pain reduction, the impact on quality of life according to the OHIP-14 score and the side effects.