Aim: Warfarin is commonly used in patients with thrombotic diseases. This study aimed to evaluate the impact of Ramadan fasting on warfarin efficacy by investigating international normalised ratio (INR) stability in medically stable patients. Methods: A retrospective observational study was conducted at King Khalid University Hospital during Ramadan 2016 on fasting adult patients aged above 18 years and receiving warfarin. The INR values during pre-Ramadan, Ramadan and post-Ramadan periods were collected after satisfying the inclusion criteria. Time within the therapeutic range (TTR) during the whole period was estimated using the conventional method. Results: In total, 101 patients were included in the study. The mean age (SD) was 55.8 ± 15.5 years, and 52.4% were females. The target INR range for 62.4% was 2-3, while 37.6% had a target INR range of 2.5-3.5. An upward trend in the proportion of patients with therapeutic INR was noticed during Ramadan (59.4%) as compared to pre-(56.4%) and post-Ramadan periods (53.5%) respectively. Additionally, the proportions of patients with supratherapeutic and sub-therapeutic INR were the highest and lowest, 23% and 24% respectively post-Ramadan as compared to other periods. Based on target INR categorisation, achieving therapeutic INR during Ramadan was more feasible with the low INR (2-3) compared to the high INR (2.5-3.5) target patients, 63.5% vs 52.6% respectively. TTR estimation revealed 62.4% and 37.6% of the patients had good and poor, respectively, anticoagulation status throughout the study period. Conclusion: Despite the changes in mean INR and proportion of patients with therapeutic INR during Ramadan compared to other non-fasting months, our results confirmed that short-term fasting during Ramadan has no significant influence on INR stability and, consequently, therapeutic efficacy in warfarin-treated medically stable patients.