Objective: The objectives of this study were to evaluate a knowledge of warfarin, type and number of drug related problems (DRPs) by pharmacists at outpatient department of Vachiraphuket Hospital.

Material and Method: The study design was descriptive cross-sectional study, convenience sampling at outpatient department of Vachiraphuket Hospital during May to June 2013.

Results: From total of 60 patients, mean age 61.6±15.1 years, the most common indication was atrial fibrillation. Patients were good knowledge about efficacy, side effect and indication but lack of knowledge about forgetting to take medicine, management of accident and serious side effect. DRPs were found 131 events, drug interaction (33.6%), adverse drug reaction (28.2%) and subtherapeutic dosage (16.0%). Adverse drug reactions were found to be dyspnea, chest pain and petechiae. Physicians and patients/ caregivers accepted DRPs and solve the problem 92.0% and 93.5% respectively. DRPs were found in patients that International Normalized Ratio (INR) out of target more than INR in target (100.0% VS 70.8%, p=0.0008)

Conclusion: Although patients were highly scores but still had problem about forgetting to take medicine, management of adverse drug reactions. Pharmacists should counseling more attention for such patients.