Overview
Warfarin is a widely used oral anticoagulant (blood thinner) that prevents the formation and growth of blood clots. It is commonly prescribed for patients with atrial fibrillation, deep vein thrombosis (DVT), pulmonary embolism (PE), or those with artificial heart valves to prevent stroke and heart attacks.

Mechanism of Action
It functions as a vitamin K antagonist. Vitamin K is essential for the hepatic synthesis of several clotting factors (Factors II, VII, IX, and X) and anticoagulant proteins (Proteins C and S). By inhibiting the enzyme vitamin K epoxide reductase, warfarin depletes the active form of vitamin K, thereby reducing the production of these active clotting factors and prolonging blood clotting time.

Dosage and Administration
Dosage is highly individualized and must be strictly guided by regular blood tests measuring the International Normalized Ratio (INR). The goal is to maintain the INR within a specific therapeutic range (usually 2.0 to 3.0, depending on the indication). It is taken once daily, preferably at the same time each day. Patients must maintain a consistent dietary intake of vitamin K (found in leafy green vegetables).

Side Effects
The primary and most serious side effect is an increased risk of severe bleeding, which can occur in any tissue or organ. Other less common side effects include bruising, hair loss, and rarely, skin necrosis or “purple toe syndrome.”

Contraindications
It is strictly contraindicated in patients with active, clinically significant bleeding, recent hemorrhagic stroke, severe liver disease, or severe hypertension. It should not be used in patients unable to comply with regular INR monitoring.

Pregnancy and Lactation
Warfarin is strictly contraindicated during pregnancy, especially in the first trimester, as it can cause fatal hemorrhage in the fetus and severe birth defects (fetal warfarin syndrome). It does not pass into breast milk in active forms and is generally considered safe for breastfeeding mothers.