BACKGROUND: Health care professionals (HCP) are known key elements of effective patient's counselling and education. For patients taking warfarin, education about the dose, side effects, and toxicity is clearly identified as a cornerstone of achieving improved health and quality of life. The study objective was to evaluate the patients' knowledge about warfarin and assess the impact of the health care professionals' counselling in enhancing patients' knowledge in achieving warfarin therapeutic outcomes. METHOD: A six-month prospective multicentered study was conducted in three hospitals, enrolling 300 patients admitted to the cardiac care unit and internal medicine departments. Patients' warfarin knowledge and INR levels were assessed before and after the clinical pharmacist counselling. The main therapeutic outcome was the impact of the clinical pharmacist-physician counselling on improving patient's education and achieving therapeutic INR level. RESULTS: A higher mean knowledge about warfarin score was found after counselling as compared to before counselling (4.82 vs 13.2; p < 0.001). Likewise, the drug dose (1.05 vs 1.88), drug toxicity (0.41 vs 1.92), drug-drug and food-drug interactions (0.02 vs 1.89), therapeutic INR and general drug knowledge scores (2.66 vs 4.68) were significantly higher after as compared to before counselling (p < 0.001 for all variables). The percentages of patients who achieved therapeutic INR levels pre/post counselling was 37.2% and 74.4% respectively (p < 0.001). CONCLUSION: Based on the study findings, HCP play a major role in enhancing patients' knowledge about the factors that affect warfarin therapeutic outcomes. This study highlights the need to establish and develop strategies for appropriate warfarin utilization in Lebanon.