Abstract : Abstract— Background: Oral chemotherapy is an alternative treatment that has high side effects that can affect cancer patient adherence and quality of life. Pharmacists have a potential role in increasing knowledge and understanding of treatment so as to increase patient adherence. Objectives: The purpose of this study was to determine the effect of pharmacist services on the level of treatment adherence of outpatient cancer patients at Arifin Achmad Hospital, Riau Province, Indonesia. Methodology: This research was an analytical descriptive study. The analysis was carried out qualitatively and quantitatively from the data questionnaire. The questionnaire was designed to measure the level of satisfaction of pharmacist services and MMAS-8 to measure the level of medication adherence. Result: The study showed that the majority of patients who received oral chemotherapy had high adherence (59.09%). The majority of patients' reasons for non-adherence were forgetting to take medication. The average level of patient satisfaction on pharmacist services was 86.59%. Conclusion: The study found that there was a correlation between the level of satisfaction and the level of adherence on the empathy dimension (p=0.011). These dimensions need to be continuously improved in order to improve oral chemotherapy adherence.