How do you adjust warfarin dose based on INR?

Warfarin dose adjustments depend on how far the INR is from the target range. For a target of 2.0–3.0: INR below 1.5 typically requires a 10–20% dose increase; INR 1.5–1.9 requires a 5–15% increase; INR 3.1–3.5 requires a 5–15% decrease; INR 3.6–4.0 requires holding one dose then decreasing by 10–15%; INR above 4.0 requires holding warfarin and possibly administering vitamin K.

What is the target INR range for warfarin?

The most common target INR range is 2.0–3.0, which applies to atrial fibrillation, DVT/PE treatment, and most other indications. A higher target of 2.5–3.5 is used for mechanical heart valves (especially mitral position) and some recurrent thromboembolism.

What is the Crowther warfarin initiation protocol?

The Crowther protocol is a validated warfarin initiation algorithm. Patients typically start with 5 mg daily (or 10 mg for younger, healthier patients). The dose is then adjusted based on INR checked on days 3–4. The 5 mg starting dose is preferred for elderly patients (>60), malnourished patients, those with liver disease, heart failure, or on interacting medications.

Which drugs interact with warfarin?

Many drugs interact with warfarin. Common potentiators (increase INR/bleeding risk) include: amiodarone, fluconazole, metronidazole, TMP-SMX, ciprofloxacin, NSAIDs, and SSRIs. Inhibitors (decrease INR) include: rifampin, carbamazepine, phenytoin, St. John's wort, and vitamin K supplements. Always check interactions when adding or stopping medications.

How often should INR be monitored on warfarin?

During initiation, INR should be checked every 2–3 days until stable. Once stable (two consecutive in-range INRs), monitoring can be extended to every 1–2 weeks, then up to every 4–6 weeks for well-established patients. Any dose change, new medication, or dietary change should prompt more frequent monitoring.

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Warfarin Dose Calculator

INR-based warfarin dose adjustment with initiation dosing, drug/food interaction alerts, and evidence-based clinical guidance.

INR-Based Dose Adjustment

Current INR Most recent INR value from lab.

Target INR Range

Current Weekly Dose (mg/week) Total mg of warfarin per week (e.g., 5 mg/day = 35 mg/week).

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Enter INR and current weekly dose to calculate adjustment.

Suggested new weekly dose—

Suggested daily dose—

Dose change—

Warfarin Initiation (Crowther Protocol)

Starting Dose

INR on Day 3 Check INR after 2–3 days of the loading dose.

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Enter day 3 INR for dose guidance.

Initiation Protocol Overview

Day	5 mg Protocol	10 mg Protocol
Day 1–2	5 mg daily	10 mg daily
Day 3	Check INR → adjust dose per algorithm
Day 4–5	Adjust based on INR; continue checking
Day 5–7	Check INR; aim for therapeutic range by days 5–7

INR Monitoring Timeline

Initiation (days 1–7): Check INR every 2–3 days
Early stabilization (weeks 1–4): Check INR every 3–7 days
Stable dose (weeks 4–12): Check INR every 1–2 weeks
Established therapy: Check INR every 4–6 weeks if stable
After any change: Recheck INR in 3–7 days (new meds, diet changes, dose adjustment)

Disclaimer: For educational purposes only. Not a substitute for clinical judgment. Warfarin dosing is highly individualized — always consider patient-specific factors including age, genetics, renal/hepatic function, nutritional status, and concomitant medications. Follow your institution's anticoagulation management protocols.

Dose Adjustment Algorithm

For target INR 2.0–3.0 (adjust ranges by +0.5 for target 2.5–3.5)

INR < 1.5 Increase by 10–20%
INR 1.5–1.9 Increase by 5–15%
INR 2.0–3.0 No change (therapeutic)
INR 3.1–3.5 Decrease by 5–15%
INR 3.6–4.0 Hold 1 dose, ↓ 10–15%
INR > 4.0 Hold warfarin, consider vitamin K

⚠️ Drug & Food Interactions

🔴 ↑ INR (Potentiators)

Amiodarone, fluconazole, metronidazole, TMP-SMX, ciprofloxacin, erythromycin, omeprazole, SSRIs, NSAIDs, acetaminophen (high dose), cranberry juice, alcohol (acute)

🔵 ↓ INR (Inhibitors)

Rifampin, carbamazepine, phenytoin, phenobarbital, cholestyramine, St. John's wort, sucralfate, vitamin K supplements, alcohol (chronic)

🟡 High Vitamin K Foods

Kale, spinach, collard greens, broccoli, Brussels sprouts, lettuce, asparagus, green tea, liver, soybean oil, canola oil

🔴 High Bleeding Risk

NSAIDs, aspirin, clopidogrel, ticagrelor, prasugrel, heparin/LMWH, SSRIs/SNRIs, fish oil (high dose), garlic supplements, ginkgo biloba

Clinical Pearls

CYP2C9/VKORC1 polymorphisms significantly affect dosing — consider pharmacogenomic testing when available.
Amiodarone is the #1 potentiator — empirically reduce warfarin dose by 30–50% when initiating.
Consistency in vitamin K intake matters more than avoidance — advise stable dietary habits.
Bridge therapy: When holding warfarin, consider heparin bridging for high-risk patients (e.g., mechanical valve, recent VTE <3 months).
Reversal: For life-threatening bleeding — 4-factor PCC (Kcentra) + vitamin K 10 mg IV.

References

Holbrook AM, et al. Systematic overview of warfarin and its drug and food interactions. Arch Intern Med. 2005;165(10):1095–1106.
Ageno W, et al. Oral anticoagulant therapy — Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: ACCP Evidence-Based Clinical Practice Guidelines. Chest. 2012;141(2 Suppl):e44S–e88S.
Witt DM, et al. Periprocedural management of antithrombotic therapy: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: ACCP. Chest. 2012;141(2 Suppl):e326S–e350S.
Stevens SM, et al. Antithrombotic therapy for VTE disease: Second Update of the CHEST Guideline. Chest. 2021;160(6):e545–e608.
Witt DM, et al. Effect of a centralized clinical pharmacy anticoagulation service on outcomes of anticoagulation therapy. Chest. 2005;127(5):1515–1522.
Crowther MA, et al. A randomized trial comparing 5-mg and 10-mg warfarin loading doses. Arch Intern Med. 1999;159(1):46–48.

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About This Tool

What Is Warfarin Dose Adjustment?

Warfarin (Coumadin) is a vitamin K antagonist anticoagulant used to prevent and treat thromboembolic events including stroke in atrial fibrillation, deep vein thrombosis (DVT), pulmonary embolism (PE), and mechanical heart valve thrombosis. Because warfarin has a narrow therapeutic index and highly variable pharmacokinetics, dosing must be individualized and guided by regular INR (International Normalized Ratio) monitoring.

How INR-Based Dose Adjustment Works

The INR measures the degree of anticoagulation — a higher INR means more anticoagulation. The goal is to maintain the INR within a target range that balances stroke/clot prevention against bleeding risk. Most indications target an INR of 2.0–3.0; mechanical heart valves (especially mitral) target 2.5–3.5. When INR falls outside the target, the weekly warfarin dose is adjusted by a percentage, typically 5–20%, depending on how far the INR deviates.

Who Needs Warfarin Dose Monitoring?

All patients taking warfarin require regular INR monitoring. This is especially critical during initiation (first 1–4 weeks), after dose changes, when new medications are started or stopped, during acute illness, and with significant dietary changes. Patients at highest risk for instability include the elderly, those with liver disease, heart failure, cancer, or multiple interacting medications.

🔑 Clinical Pearls

Time in Therapeutic Range (TTR) is the key quality metric — aim for TTR >65%. Below this, consider switching to a DOAC if eligible.
Warfarin has a 36–42 hour half-life — dose changes take 3–5 days to reach new steady state. Don't recheck INR too soon after adjustment.
Genetic variability: CYP2C9 poor metabolizers and VKORC1 variants may need 50–75% lower doses. Consider genotyping in patients with unexpected sensitivity.
Consistency over avoidance: Patients should not avoid vitamin K–rich foods; rather, they should maintain a consistent diet so the warfarin dose can be calibrated accordingly.
The biggest interactor is amiodarone — when starting amiodarone, empirically reduce warfarin by 30–50% and check INR in 3–5 days.

Warfarin Initiation: The Crowther Protocol

The Crowther protocol provides evidence-based guidance for initiating warfarin therapy. A 5 mg starting dose is recommended for most patients, particularly the elderly (>60), malnourished patients, those with liver disease or heart failure, and patients on interacting drugs. A 10 mg loading dose may be used in younger, otherwise healthy patients without risk factors for warfarin sensitivity. INR should be checked on day 3, and the dose adjusted according to a nomogram.

When to Consider Alternatives to Warfarin

DOACs (apixaban, rivaroxaban, edoxaban, dabigatran) are preferred over warfarin for non-valvular AF and VTE in most patients per current guidelines. Warfarin remains the only appropriate anticoagulant for mechanical heart valves, moderate-to-severe mitral stenosis, and antiphospholipid syndrome (triple-positive). Consider switching to a DOAC if time in therapeutic range is consistently below 65%.

Key References

Holbrook AM, et al. Systematic overview of warfarin and its drug and food interactions. Arch Intern Med. 2005;165(10):1095–1106.
Ageno W, et al. Oral anticoagulant therapy — ACCP Evidence-Based Guidelines. Chest. 2012;141(2 Suppl):e44S–e88S.
Stevens SM, et al. Antithrombotic therapy for VTE disease: CHEST Guideline Update. Chest. 2021;160(6):e545–e608.
Crowther MA, et al. A randomized trial comparing 5-mg and 10-mg warfarin loading doses. Arch Intern Med. 1999;159(1):46–48.
Witt DM, et al. Clinical pharmacy anticoagulation service outcomes. Chest. 2005;127(5):1515–1522.

Formula last verified: February 2026