Warfarin is one of the most commonly prescribed oral anticoagulants worldwide. It has been used for decades in the prevention and treatment of thromboembolic disorders, especially in patients requiring long-term anticoagulation.
For students, warfarin is a high-yield drug because of its narrow therapeutic index, need for regular INR monitoring, and multiple drug and food interactions.
This article explains what warfarin is, how it works, when it is used, how it is monitored, and its major adverse effects, in simple and exam-relevant language.
What Is Warfarin?
Warfarin is an oral anticoagulant belonging to the class of vitamin K antagonists.
It prevents blood clot formation by reducing the synthesis of vitamin K–dependent clotting factors.
Unlike heparin, warfarin has a delayed onset of action and is mainly used for long-term anticoagulation.
Classification of Warfarin
| Category | Description |
|---|---|
| Drug class | Vitamin K antagonist |
| Route | Oral |
| Onset of action | 2–3 days |
| Duration | Long |
| Use | Chronic anticoagulation |
Mechanism of Action of Warfarin
Warfarin works by inhibiting vitamin K epoxide reductase in the liver.
Step-by-Step Mechanism
1. Vitamin K is required to activate clotting factors2. Warfarin blocks vitamin K recycling
3. This reduces synthesis of:
- Factor II (Prothrombin)
- Factor VII
- Factor IX
- Factor X
- Protein C and Protein S
Key Exam Line
Warfarin reduces synthesis of vitamin K–dependent clotting factors (II, VII, IX, X).
Why Warfarin Has a Delayed Onset
Warfarin does not affect existing clotting factors.
It only prevents formation of new clotting factors, so anticoagulation appears after old factors are cleared, usually in 48–72 hours.
This is why heparin bridging is often used initially.
Clinical Uses of Warfarin
Warfarin is mainly used for long-term prevention of thrombosis.
Major Indications
- Deep vein thrombosis (DVT)
- Pulmonary embolism (PE)
- Atrial fibrillation (stroke prevention)
- Mechanical heart valves
- Post-myocardial infarction (selected cases)
- Prevention of recurrent thromboembolism
Dose and Administration
General Dosing Principles
- Given orally
- Dose is individualized
- Adjusted according to INR values
| Phase | Typical Dose |
|---|---|
| Initial | 2–5 mg/day |
| Maintenance | Based on INR |
| Elderly | Lower doses required |
Sudden dose changes should be avoided.
Monitoring of Warfarin Therapy
INR (International Normalized Ratio)
INR is used to measure anticoagulant effect.
| Indication | Target INR |
|---|---|
| DVT / PE | 2.0–3.0 |
| Atrial fibrillation | 2.0–3.0 |
| Mechanical heart valve | 2.5–3.5 |
Regular monitoring is mandatory to prevent bleeding or thrombosis.
Adverse Effects of Warfarin
Common Side Effects
- Bleeding
- Easy bruising
- Nosebleeds
- Hematuria
Serious Adverse Effects
1. Hemorrhage
- Most common and dangerous complication
- Risk increases with high INR
2. Warfarin-Induced Skin Necrosis
- Rare but severe
- Occurs early due to rapid fall in Protein C
3. Teratogenicity
- Causes fetal abnormalities
- Contraindicated in pregnancy
Contraindications of Warfarin
Warfarin should not be used in:
- Pregnancy
- Active bleeding
- Severe liver disease
- Bleeding disorders
- Uncontrolled hypertension
- Recent major surgery (relative)
Drug and Food Interactions
Drug Interactions (Increase Bleeding Risk)
- Aspirin
- NSAIDs
- Antibiotics (e.g., metronidazole)
- Amiodarone
Food Interactions
Foods rich in vitamin K reduce warfarin effect:
- Spinach
- Broccoli
- Cabbage
- Green leafy vegetables
Consistency in diet is more important than avoidance.
Antidote for Warfarin
Vitamin K (Phytonadione)
- Used to reverse warfarin effects
- Oral or IV route
- Fresh frozen plasma (FFP) used in emergencies
Warfarin vs Heparin (Quick Comparison)
| Feature | Warfarin | Heparin |
|---|---|---|
| Route | Oral | IV / SC |
| Onset | Delayed | Immediate |
| Monitoring | INR | aPTT |
| Pregnancy | Unsafe | Safe |
| Antidote | Vitamin K | Protamine sulfate |
Special Precautions
- Regular INR monitoring
- Avoid alcohol excess
- Inform doctor before surgery
- Maintain consistent diet
- Educate patient about bleeding signs
High-Yield Exam Points
- Oral vitamin K antagonist
- Delayed onset of action
- Narrow therapeutic index
- Requires INR monitoring
- Contraindicated in pregnancy
- Antidote: vitamin K
FAQs About Warfarin
1. What is warfarin mainly used for?
Warfarin is used for long-term prevention of blood clots, especially in atrial fibrillation, DVT, and mechanical heart valves.
2. Why is INR monitoring required with warfarin?
Because warfarin has a narrow therapeutic range, INR monitoring helps prevent bleeding or clot formation.
3. Can warfarin be used in pregnancy?
No. Warfarin is teratogenic and contraindicated during pregnancy.
4. Why is heparin given before warfarin?
Warfarin has a delayed onset, so heparin bridging provides immediate anticoagulation.
5. What foods interfere with warfarin?
Foods rich in vitamin K, such as green leafy vegetables, can reduce its effect.
6. What is warfarin-induced skin necrosis?
A rare complication caused by rapid fall in Protein C levels, leading to skin ischemia.
7. What is the antidote for warfarin overdose?
Vitamin K is the specific antidote.
8. Does warfarin dissolve clots?
No. Warfarin prevents formation of new clots, but does not dissolve existing ones.
