At Anjo Kosei Hospital, patients receive injectable medication for diabetes treatment and devices for self-monitoring of blood glucose as in-hospital prescriptions. Pharmacists manage prescriptions according to established protocols when there is over- or under-prescribing. In this study, we retrospectively examined outpatient prescriptions for patients attending endocrinology and diabetology from January 2014 to June 2015 (before protocol implementation) and from July 2015 to December 2020 (after protocol implementation) to evaluate the usefulness of protocol-based supporting prescription by pharmacists. Changing prescriptions based on the protocol simplified prescription queries in 661 of 721 cases (91.7%), and significantly decreased the rate of unplanned doctor visits after the introduction of the protocol (p<0.05). Optimization of prescriptions through protocol-based assistance in prescribing is expected to reduce the burden on physicians, patients, and pharmacists by reducing the number of formal inquiries for questionable prescriptions and the number of unplanned doctor visits.