Background: Patients must adhere to their prescribed warfarin regimens and regularly monitor the anticoagulation effect to maintain therapeutic levels. The ability to evaluate regimen adherence accurately is crucial to the success of patient coaching. However, prevention of thromboembolic events is challenging when the association between medication adherence and the actual amount of anticoagulant agent taken cannot be determined. Purpose: This study used self-reported medication taken and prescription refills to (a) verify warfarin medication adherence and (b) assist clinicians to determine the discrepancy between medication adherence and anticoagulant control efficacy. Methods: This study was conducted at a national-university affiliated hospital in Yilan County, Taiwan. Structured questionnaires and medical record reviews were adopted. A 100-point visual analog scale was used to measure the reported adherence of participants, whereas medication refill adherence was compared against self-reported adherence. Finally, degree of adherence was evaluated based on time in therapeutic range. Results: This study included 192 participants. Half (n = 94, 49%) were women, and the mean age was 69.6 years. Mean scores were 92.2% for the visual analog scale and 87.3% for medication refill adherence. Medication adherence correlated significantly with age, as reflected in the visual analog scale scores (p < .05). The participants who were receiving poly-pharmacy with five types of medicines or less attained higher visual analog scale scores, whereas participants who were on warfarin for 2–12 months exhibited higher medication refill adherence. Time in therapeutic range correlated negatively with age, although stability improved with therapy duration. Conclusions/Implications for Practice: Despite their high self-reported adherence levels, patients of advanced age require careful monitoring of their time in therapeutic range. The participants in this study who were on warfarin for a relatively longer time exhibited higher stability in the therapeutic range, despite their low medication refill adherence. The results of this study suggest that patient age, duration of warfarin therapy, and poly-pharmacy are factors associated with medication adherence. The findings may facilitate future assessments of warfarin adherence in patients as well as the implementation of more effective clinical nursing procedures and management practices.