Formulary
Warfarin: Uses, Dosing, Side Effects and Indian Brand Names
Warfarin is a vitamin K antagonist with a narrow therapeutic index, monitored by INR, essential for anticoagulation in mechanical heart valves and widely used in India for AF and VTE due to lower cost than DOACs.
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Warfarin: Uses, Dosing, Side Effects and Indian Brand Names
Warfarin is a vitamin K antagonist with a narrow therapeutic index, monitored by INR, essential for anticoagulation in mechanical heart valves and widely used in India for AF and VTE due to lower cost than DOACs.
NEET PG High-Yield: VKORC1 enzyme target. Factors affected: II, VII, IX, X and proteins C and S (mnemonic: 1972 -- factors 10, 9, 7, 2). Factor VII shortest half-life (6h) -- PT/INR rises first. Protein C depletion causes paradoxical prothrombotic state -- warfarin skin necrosis. Must bridge with heparin for 5 days. CYP2C9 and VKORC1 pharmacogenomics. Teratogenic (Category X). INR target 2-3 (most) or 2.5-3.5 (mechanical mitral). Antidote: vitamin K (slow), FFP/PCC (immediate). Rifampicin is the most important inducer.
Clinical overview
Warfarin remains the only oral anticoagulant approved for mechanical heart valve prostheses and is still widely used in India for atrial fibrillation, venous thromboembolism, and valvular heart disease, largely because direct oral anticoagulants (DOACs) are significantly more expensive and lack an indication for mechanical valves. Warfarin has a notoriously narrow therapeutic index, with anticoagulation monitored using the International Normalised Ratio (INR). Target INR is 2.0-3.0 for most indications and 2.5-3.5 for mechanical mitral valves and recurrent thromboembolism. Its clinical use is complicated by extensive drug and food interactions, genetic polymorphisms affecting metabolism (CYP2C9) and sensitivity (VKORC1), and the paradoxical initial prothrombotic state caused by early depletion of protein C (shortest half-life among vitamin K-dependent proteins). This prothrombotic window is why warfarin must be bridged with heparin for the first 5-7 days, and why loading with high doses can cause warfarin-induced skin necrosis, particularly in protein C deficiency. Vitamin K1 (phytomenadione) reverses warfarin effect in 12-24 hours; for life-threatening bleeding, IV vitamin K plus 4-factor prothrombin complex concentrate (PCC) or fresh frozen plasma provides immediate reversal. The anticoagulant effect of warfarin is measured by INR, which is the ratio of the patient's PT to a reference PT, raised to the power of the ISI (International Sensitivity Index).
Pharmacological class
Warfarin belongs to the Vitamin K Antagonists (Oral Anticoagulants) class. Inhibits vitamin K epoxide reductase (VKORC1), preventing the regeneration of reduced vitamin K. This blocks the gamma-carboxylation of vitamin K-dependent clotting factors (II, VII, IX, X) and anticoagulant proteins C and S. Factor VII has the shortest half-life (6 hours), so PT/INR rises first, but full anticoagulant effect requires depletion of factor II (prothrombin, half-life 60-72 hours), taking 5-7 days.
Indian brand names and formulations
Available as: Warf (Cipla), Acitrom (Abbott) -- note: Acitrom contains acenocoumarol, not warfarin, Uniwarfin (Unichem), Sofarin (Dr. Reddy's).
Tablets: 1 mg, 2 mg, 3 mg, 5 mg (colour-coded in some brands). Note: Acenocoumarol (Acitrom) is a separate coumarin anticoagulant popular in India -- shorter acting, twice-daily dosing possible; not interchangeable with warfarin on a mg-for-mg basis.
Regulatory status
Warfarin is classified under Schedule H in India under the Drugs and Cosmetics Act, 1940. Prescription-only. Requires regular INR monitoring. Patients must carry an anticoagulant card.
Indications
- Mechanical heart valve prostheses (the ONLY oral anticoagulant approved)
- Atrial fibrillation (stroke prevention; DOACs preferred in non-valvular AF if affordable)
- Deep vein thrombosis and pulmonary embolism (treatment and secondary prevention)
- Antiphospholipid syndrome
- Rheumatic mitral stenosis with atrial fibrillation
- Left ventricular thrombus post-MI
Dosing
Initiation: 2-5 mg once daily (avoid large loading doses). Overlap with LMWH/heparin for minimum 5 days AND until INR is therapeutic (2.0-3.0) for at least 24 hours. Dose adjusted based on INR. Target INR: 2.0-3.0 (most indications), 2.5-3.5 (mechanical mitral valve, recurrent VTE). In India, patients often need lower doses (2-4 mg) compared to Western populations due to VKORC1 polymorphisms.
Contraindications
- Pregnancy (Category X -- crosses placenta, causes warfarin embryopathy: nasal hypoplasia, stippled epiphyses in 1st trimester; CNS abnormalities in 2nd/3rd trimester)
- Active major bleeding
- Severe uncontrolled hypertension
- Severe hepatic disease with coagulopathy
- Recent CNS or ophthalmic surgery
- Non-compliant patients or those without access to INR monitoring
Adverse effects
- Bleeding (most important -- intracranial, GI, genitourinary; risk increases exponentially above INR 4.5)
- Warfarin-induced skin necrosis (rare; occurs days 3-8 due to paradoxical protein C depletion; associated with protein C deficiency)
- Purple toe syndrome (cholesterol microembolisation, rare)
- Teratogenicity (warfarin embryopathy)
- Alopecia (uncommon)
Drug interactions
- CYP2C9 inhibitors (amiodarone, fluconazole, metronidazole, cotrimoxazole): increase warfarin levels, increase INR and bleeding risk
- CYP2C9/CYP3A4 inducers (rifampicin, carbamazepine, phenytoin, St John's wort): decrease warfarin effect, require dose increase
- Vitamin K-rich foods (green leafy vegetables): reduce anticoagulant effect; maintain CONSISTENT intake, do not avoid completely
- Aspirin and NSAIDs: increased bleeding risk (pharmacodynamic interaction + GI mucosal damage)
- Antibiotics: broad-spectrum antibiotics reduce gut flora vitamin K synthesis, increasing INR unpredictably
- Alcohol (binge): acutely inhibits metabolism, increases INR; chronic alcohol use induces metabolism, decreases INR
Pregnancy and lactation
Category X. Absolutely contraindicated, especially in the first trimester (warfarin embryopathy: nasal hypoplasia, chondrodysplasia punctata at 6-12 weeks). In the second and third trimester, CNS abnormalities and foetal haemorrhage can occur. For pregnant women requiring anticoagulation (e.g., mechanical valves), LMWH or unfractionated heparin is used throughout or warfarin may be used in the second trimester only under specialist supervision in very high-risk patients.
Exam-style clinical scenario
A patient with a mechanical mitral valve presents with INR of 8.5 and gum bleeding but no major haemorrhage. Management? Hold warfarin, give oral vitamin K 2.5-5 mg (IV for more rapid correction), recheck INR in 12-24 hours. Do NOT give FFP or PCC for non-life-threatening bleeding. Resume warfarin at a lower dose once INR is therapeutic.
Cost in India
Rs 10-30 for a strip of 10 tablets (various strengths). INR testing costs Rs 100-300 per test at private labs.
Clinical governance
Author: MedNext Editorial Team. Clinical reviewer: Awaiting clinical review. Jurisdiction: India (Drugs and Cosmetics Act, 1940). Sources: CIMS India, Indian Pharmacopoeia. Publication state: Awaiting clinical review. Correction: Report errors at support@mednext.academy.
Frequently Asked Questions
Why must warfarin be bridged with heparin?
Warfarin inhibits synthesis of both procoagulant factors (II, VII, IX, X) and anticoagulant protein C. Protein C has the shortest half-life (~8 hours) among these, so it is depleted FIRST, creating a transient prothrombotic state during the first few days of warfarin therapy. Concurrent heparin provides immediate anticoagulation until all vitamin K-dependent factors reach new steady state (5-7 days).
How do VKORC1 and CYP2C9 polymorphisms affect warfarin dosing?
VKORC1 polymorphisms affect the sensitivity of the target enzyme -- patients with certain variants (common in Asians) are more sensitive and need lower doses. CYP2C9 polymorphisms (CYP2C9*2 and *3) reduce warfarin metabolism, increasing drug levels and bleeding risk. Together, these polymorphisms can account for a 3-5 fold difference in dose requirements between patients.
Why should patients on warfarin maintain consistent vitamin K intake rather than avoid it?
Vitamin K competes with warfarin at the VKORC1 enzyme. If a patient on a stable warfarin dose suddenly increases or decreases vitamin K intake, the INR will fluctuate unpredictably. The advice is to eat a consistent amount of green leafy vegetables day to day, so the warfarin dose can be calibrated against a stable dietary baseline.
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