Objective: In patients with specific backgrounds, comprehensive identification of health problems and proactive pharmacist intervention are crucial to providing safe and effective medical care. However, there are insufficient reports on chemotherapy regimen selection and supportive care management in patients taking immunosuppressants. In this study, to circumvent adverse events, pharmacists intervened with a patient administering tacrolimus (TAC) using known information, focusing on multiple factors attributable to the patient in addition to drug interactions.Methods: The patient was a male in their 70s who received palliative chemotherapy for gastric cancer during their dermatomyositis treatment with TAC. Pharmaceutical support for cancer chemotherapy was provided using the following four procedures: (1) Patient information was collected from interviews and electronic medical records to identify patient-specific problems; (2) Basic pharmacological information was collected from tertiary sources, focusing on the interaction between TAC and aprepitant (APR). Furthermore, clinical reports were collected, and the pharmacokinetic drug interaction significance classification system was used for quantitative predictions; (3) The information obtained in steps 1) and 2) was evaluated, and comprehensive proposals linked to the patient information were presented; (4) Adverse events, TAC blood level, and patient outcomes were monitored after treatment initiation.Results: A chemotherapy regimen consisting of S-1/oxaliplatin therapy without APR was selected. The adverse effects were controllable, and the treatment was completed without many adverse events. Meanwhile, TAC adherence was unaffected by cancer chemotherapy, and the TAC blood concentration or dose ratios were controlled within the same range as previously reported.Conclusion: In cancer chemotherapy, for cases with limited evidence or information, comprehensive pharmaceutical support was provided using known patient information, considering multiple patient factors. This report is beneficial as an example of supportive care management by a pharmacist and contributes to providing optimal service in cases with specific backgrounds.