Study participants are adult patients of the Department of Oral Medicine, School of Dental Medicine in Zagreb, with a clinically and histologically proven OLP or RAS in the acute stage of the disease.The treatment outcome of the OLP patients will be measured by clinical improvement and subjective symptomatic relief. The outcome of RAS patients treated by NAVS naphthalan or by betamethasone will be measured clinically by the decrease in number and size of lesions as well as by subjective symptomatic relief over treatment period. One member of the team, who will not evaluate the therapeutic effect, will took care of the allocation of test and control preparations. At the end of the study, a randomization code will be opened and statistically analysed. In both clinical and subjective domains, of both clinical conditions, the improvement rate will be measured by comparing these readings, as the percentual reduction of clinical scores and symptoms. Since the data will not be normally distributed, methods of nonparametric statistics will be used: Wilcoxon test for dependent and Mann-Withney test for independent samples. Baseline intergroup differences will be assessed by Mann-Withney test. For the interpretation of the average values, medians and interquartile ranges (IQR) will be used. Fisher exact test will be used to compare gender representation among the groups. Statistical analysis will be performed using MedCalc Software 13.0.0.0 (Acacialaan 22, 8400 Ostend, Belgium). P value lower then 0.05 (p< 0.05) will be considered statistically significant.