Objective: Pharmacists at insurance pharmacies play an important role in the pharmaceutical care of outpatients receiving cancer chemotherapy. This study aimed to clarify the actual status of insurance pharmacies' involvement in cancer chemotherapy and associated issues, based on an analysis of prescription inquiries made to doctors by pharmacists at an insurance pharmacy.Design: This was a retrospective observational study.Methods: The data was collected in one insurance pharmacy, which received prescriptions mainly from Gunma Prefectural Cancer Center. Among 2, 258 inquiries recorded from January 2015 to May 2018, inquires related to oral anticancer drugs or supportive care medicine were extracted. The frequency of inquiries for each item, or the frequencies of factors that lead to inquiries were calculated. Inquiries considered to have potentially led to the prevention or avoidance of adverse drug reactions (ADRs), so-called “preavoidance” inquiries, were also extracted.Results: Four hundred and forty inquiries related to 20 oral anticancer drugs were included in the analysis. The prescriptions were changed after 92.7% of all prescription inquiries. Prescription inquiries for drugs with rest periods were more frequent than those for drugs without rest periods. The most common inquiries were about the medication schedules stated on the prescription, followed by inquiries about supportive care drugs. Approximately 60% of the pharmacy inquiries were related to“pre-avoidance”inquiries. Most of the pre-avoidance inquiries concerned prevention of ADRs, though these inquiries also contributed to“reduction or avoidance of mental anxiety”. The prescription inquiries were triggered by information collected by pharmacists from patient interviews and from medication histories.Conclusion: Our findings suggest that inquiries to the prescribing doctors by pharmacists at insurance pharmacies contribute significantly to the appropriate use of anticancer drugs.