For improving the QOL of patients diagnosed with cancer, early palliative care is recommended, aiming to minimize pain and opioid-induced side effects. Herein, we evaluated the effect of continuous interventions for pain management and opioid-induced side effects in outpatients with cancer. Pharmacists continuously performed interventions on patients on their hospital visits, starting from the first visit for opioid introduction to intervention via telephone. We recorded their pain patterns and intensities, use of rescue doses, and types and degrees of side effects during these interventions. The physicians were suggested appropriate recommendations for increased doses or alternative opioids when the pharmacists considered the analgesic dose should be titrated. During the study period, palliative care pharmacists conducted 105 interviews for 27 patients (male: 19 and female: 8) with cancer pain. Pain intensities significantly decreased after the pharmacists’ continuous intervention, including those from telephone interviews, with their appropriate recommendations and increased opioid doses. Side effects such as nausea and constipation increased or remained unaffected even after the intervention, likely due to the increased opioid doses. Approximately 90% of recommendations for pain control were accepted by the physicians and helped to control the pain intensities. Before starting physician consultations, pharmacists informed the patients that adequate pain control and side effect management were achievable through regular interviews, wherein patient symptoms were monitored and patients received detailed explanations of pharmaceutical care and courteous and continuous counseling.