How can I adjust a patient on warfarin with a target INR of 2.5‑3.0 who currently takes 4 mg daily except two days per week when only a half tablet (≈2 mg) is taken, to achieve an average daily dose of about 4.2 mg using only 4 mg tablets?

Practical Warfarin Dosing Schedule to Achieve 4.2 mg Daily Average

To achieve an average daily dose of 4.2 mg per week (29.4 mg weekly) using 4 mg tablets, take 4 mg daily for 5 days per week and 4.5 mg (one and one-eighth tablets) on 2 days per week.

Calculating the New Weekly Dose

Your patient currently takes:

• 4 mg × 5 days = 20 mg
• 2 mg × 2 days = 4 mg
• Current weekly total = 24 mg

Target weekly dose for 4.2 mg daily average:

• 4.2 mg × 7 days = 29.4 mg per week

This represents a 22.5% increase from the current regimen, which is appropriate for an INR that requires upward adjustment. 1, 2

Recommended Dosing Schedules (Choose One)

Option 1: Simplest Schedule

• Monday through Friday: 4 mg daily (one tablet)
• Saturday and Sunday: 4.5 mg (one tablet plus one-quarter tablet)
• Weekly total: 29 mg

Option 2: Alternative Pattern

• 5 days per week: 4 mg (one tablet)
• 2 days per week: 5 mg (one and one-quarter tablets)
• Weekly total: 30 mg

Option 2 provides slightly more anticoagulation (30 mg vs 29 mg weekly) and may be easier if cutting tablets into eighths is difficult. 3

Tablet-Splitting Technique

• To create a 4.5 mg dose: Score one 4 mg tablet in half, then cut one half in half again to create a quarter-piece. Take one whole tablet (4 mg) plus one quarter-piece (0.5 mg). 3

• To create a 5 mg dose: Cut one 4 mg tablet in half and take one whole tablet (4 mg) plus one half-tablet (2 mg) = 6 mg, which is too much. Instead, take one whole 4 mg tablet plus one-quarter of another tablet (1 mg) = 5 mg. 3

Critical Monitoring Requirements

• Recheck INR within 1–2 weeks after implementing the new dosing schedule to confirm appropriate response and avoid overshooting the therapeutic range of 2.5–3.0. 1, 2

• Continue monitoring INR every 1–2 weeks until two consecutive therapeutic values (2.5–3.0) are achieved, then extend to weekly monitoring for one month. 1, 2

• Once stable for one month in the therapeutic range, monitoring intervals can be extended to every 4–12 weeks. 2

Important Caveats

• Avoid dose reductions >20% or increases >20% for routine adjustments, as larger changes frequently cause the INR to overshoot or undershoot the target range. 4

• Investigate potential causes of the subtherapeutic INR before making the dose change: medication changes (especially antibiotics), dietary changes in vitamin K intake, intercurrent illness, changes in liver or renal function, or medication non-adherence. 1, 2

• Elderly patients (>65 years) are more sensitive to warfarin and may require smaller dose adjustments; if your patient is elderly, consider starting with Option 1 (29 mg weekly) rather than Option 2 (30 mg weekly). 1

• Maintain consistent vitamin K intake throughout the week, as fluctuations in dietary vitamin K (from green leafy vegetables) can cause INR variability that undermines dose adjustments. 2

When to Hold or Reduce the Dose

• If the follow-up INR is 3.1–3.5, decrease the weekly dose by 10% (reduce to approximately 26–27 mg weekly). 1, 5

• If the follow-up INR is >3.5 but <5.0, hold warfarin for one dose, then restart at a dose 10–20% lower than 29.4 mg weekly. 1, 5

• Do not administer vitamin K for an INR <5.0 in the absence of bleeding unless the patient has high-risk bleeding factors (age >65–75 years, prior bleeding history, concurrent antiplatelet therapy, renal failure, or alcohol use). 1, 5