Background The outcomes of most therapeutic modalities for recurrent aphthous ulcer (RAU) are still unsatisfactory. Aim To evaluate lactic acid 5% mouth wash vs Kenalog in Orabase for treatment and prophylaxis of RAU. Patients/Methods Forty cases with early‐onset idiopathic RAU were enrolled in this study. Patients were divided into two equal groups; group A patients had used Kenalog in Orabase twice daily, and group B patients had used lactic acid 5% mouth wash 3 times daily. All patients had used the therapy for 1‐2 weeks according to patients' clinical response that was evaluated according to oral clinical manifestations index (OCMI); before therapy, during course of treatments and in follow‐up visits. Results At the ends of both first and second weeks, from beginning of therapy, OCMI was reduced more in group B patients than in group A with statistically significant results. These results revealed that group B achieved more reduction in the size, pain, and healing time of RAU. During the follow‐up period, group A showed 40% recurrence rate while group B showed 5% only. Conclusions Lactic acid 5% mouth wash is natural, safe, and effective so it is better alternative to corticosteroids for treatment and prophylaxis of RAU without any side effects.