In a retrospective study which we have done (to be published), we found that 73% of the patients who received Vitamin B12 there were no recurrent incidents of canker sores in the mouth. Assuming that among the patients that do not receive the Vitamin B12 treatment the frequency of the disease will be at least 90%, and that in the study group we expect that not more than 60% of the patients will continue to suffer from recurrent canker sores, in order to achieve a confidence level of 95% and a power of 80%, we need to recruit about 38 patients in the study group and 38 patients in the placebo group. With the assumption that the drop-out rate will be 30%, we need to recruit about 50 patients in each group.