Background: This study aimed to investigate whether the different lunar phases have any effect on prothrombin time and international normalized ratio levels in patients diagnosed with deep vein thrombosis who are undergoing warfarin therapy. Understanding potential external influences, such as lunar phases, on anticoagulant therapy could provide new insights into optimizing patient outcomes. Material and Methods: Data were retrospectively analyzed from 1418 international normalized ratio records of 124 patients with deep vein thrombosis who received warfarin therapy between March 2017 and August 2023. The lunar phase data were integrated with the corresponding international normalized ratio results, and statistical analyses were performed. The Jonckheere-Terpstra test for ordered alternatives was used to detect trends, while the Pettitt test assessed the presence of change points in the data. Results: An increasing trend in international normalized ratio values was observed during the third quarter lunar phase. While this trend suggested a potential tendency toward statistical significance, it did not reach the threshold for significance (Jonckheere-Terpstra test: Z = 0.916, P = 0.360) and no distinct change point was detected (Pettitt test: U = 6, P > 0.999). Conclusion: Although not statistically significant, this retrospective cohort study demonstrated a potential shift toward elevated prothrombin time levels during the third quarter lunar phase. While the results do not provide definitive evidence, they highlight the need for further large-scale, multicenter studies to explore the potential relationship between lunar phases and fluctuations in international normalized ratio values in patients undergoing warfarin therapy. Such research may provide clinicians with additional insights into managing anticoagulant treatments effectively.