Seirogan, a wood creosote, is a nonprescription drug used to treat diarrhea. However, reports of its clinical use are rare. Here, we report on the efficacy of wood creosote (3 capsules daily) for the alleviation of diarrheal symptoms in 148 patients from 10 clinics in Japan. The wood creosote capsules were classified as remarkably effective (44 patients), effective (71 patients), and partially effective (13 patients) based on the degree of alleviation of diarrheal symptoms that were induced by a variety of causes. The antidiarrheal efficacy of the capsules did not differ between males and females, and young patients (21–30 years) showed greater improvement in diarrheal symptoms than did old patients (> 61 years). Although this report is based on the re-evaluation of old data that had been preserved by our company, the effectiveness and range of symptoms that were treatable with wood creosote has likely remained unchanged. To the best of our knowledge, this is the first public report on the clinical effectiveness of wood creosote capsules for the treatment of a wide range of diarrheal symptoms.