PurposeSeveral factors are known to be significantly associated with a low completion rate of 1-year adjuvant S-1 monotherapy for gastric cancer. The present study investigated whether or not the specialties of physicians conducting adjuvant S-1 monotherapy affect the completion rate.MethodsA total of 437 patients who underwent curative gastrectomy followed by adjuvant S-1 monotherapy for pathological stage II or III gastric cancer between 2008 and 2013 were retrospectively analyzed. Factors affecting completion of adjuvant S-1 monotherapy, including the physicians (medical oncologists or surgeons) administering S-1, were evaluated by a multivariate analysis. The relationship between patient factors and physicians was analyzed regarding the cumulative incidence of discontinuation. The number of times the dose was reduced, the schedule changed, or administration was suspended or delayed in patients completing adjuvant S-1 monotherapy was also counted.ResultsThe multivariate analysis showed that old age (≥ 65 years old), excess body weight loss (≥ 15%), and surgeons were independently associated with discontinuation. In older patients, the cumulative incidence of discontinuation by medical oncologists was significantly lower than that by surgeons. Medical oncologists ensured that older patients continued S-1 by frequent suspension or a delay in each course.ConclusionsMedical oncologists may facilitate completion of adjuvant S-1 monotherapy.