We previously reported the development and clinical efficacy of a 2% aspirin oral ointment and 2% ethenzamide oral ointment as hospital preparations for painful lesions of the oral mucosa. This study investigated methods of preparing a more stable oral ointment with a more effective analgesic action, using diflunisal, another salicylic acid derivative, with an analgesic effect stronger than that of aspirin. A two-percent diflunisal oral ointment was prepared similarly to the aspirin ointment using plastibase and CMC-Na as the ointment base. From the results of spreadability measurement, a CMC-Na content of 20% was considered appropriate. The stability of diflunisal in 2% diflunisal oral ointment stored at 5 degrees C, 20 degrees C and 30 degrees C, was determined using HPLC, and a high stability of diflunisal at room temperature for more than 100 days was confirmed. We also investigated its antinociceptive effect using the Randall-Selitto paw pressure test in rats, which showed that 2% diflunisal oral ointment was as effective as 2% aspirin oral ointment. On clinical application of 2% diflunisal oral ointment to 8 patients with painful oral mucous diseases, it was found to be significantly (p = 0.014) more effective than 2% aspirin oral ointment. The results of this study demonstrated that 2% diflunisal oral ointment is a clinically useful analgesic for painful oral lesions.