Warfarin Prescribing and Counselling

NB- Make sure to follow local guidance on warfarin prescribing (there are often slight differences- not all guidance is the same)


  • Venous thromboembolism (DVT/PE)
  • Atrial Fibrillation (and pre/post-cardioversion)
  • Valvular heart disease and valve replacements
  • Mitral stenosis/regurgitation (with AF or history of systemic embolism/left atrial thrombus/enlarged left atrium)
  • Dilated cardiomyopathy


  • Haemorrhagic stroke
  • Bleeding disorders (including liver failure/renal failure)
  • Potential bleeding lesions e.g. peptic ulcer, varices, aneurysm, proliferative retinopathy, recent trauma
  • Uncontrolled severe hypertension (SBP>200mmHg; DBP>120mmHg)
  • Pregnancy

Counselling patients about warfarin

  • WIPE (Wash hands, Introduce self, Check Patient details, Explain why you are there)
    • It is good to check understanding of warfarin, patient’s condition, risks and benefits, any contraindications (double check consent)
    • Explain
      • Side effects e.g. Bleeding/bruising, rash, alopecia, nausea/diarrhoea
        • Explain when to contact doctor/medical attention e.g. blood in urine, in vomit, in sputum/cough, excessive nose bleeds, excessive PV bleeding, prolonged wound bleeding (generally >5 mins for small cuts)
      • Drug interactions
      • Monitoring
  • Explain how to take/compliance
    • Try to take at same time every day (e.g. 6pm in the evening)
      • Take missed dose ASAP (within 6 hours of missed dose)- do not double up doses
    • Explain the different warfarin doses/tablets (colours) and how to make up the dose
    • Explain the yellow book (or equivalent) and alert card
    • Explain the importance of not running out of tablets.
    • Explain duration of therapy
      • For single VTE (with explanation)- 3 months; (without explanation)- 6 months
      • For recurrent VTE – usually long term
      • For AF – as long as the condition persists
      • Valve replacements- lifelong
      • Cardioversion – 4 weeks before and after
    • Explain monitoring- measure INR initially every few days/every week; if the INR is stable on a good maintenance dose, the frequency can be reduced
  • Explain lifestyle changes
    • Advise against contact sports
    • Advise re certain foods e.g. broccoli, sprouts, cabbage, liver, pork, cranberry juice, grapefruit juice- try to limit/avoid
    • If a heavy drinker, recommend cutting down (make sure to check LFTs/clotting prior to initiation).  Ideally, minimal alcohol intake.
    • Discuss appropriate contraception- in general try to avoid oestrogen containing preparations.  Reinforce importance of contraception and warfarin in pregnancy.
    • Advise about falling ill/going to hospital- letting staff know about the prescription.  Also re: drug interactions (don’t take aspirin, seek advice before starting herbal/alternative preparations, inform doctor before starting any new drugs/change of dose- may require more frequent INR monitoring)
      • Also important to inform your dentist
  • Explain initiation
    • 2 kinds- fast/rapid (for acute VTE/valve replacement) and slow (for AF/valve disease)
    • Fast initiation
      • Usually used in combination with LMWH (for 4 days minimum and until INR>2)
      • ‘Loading dose’ of warfarin is usually 5mg (can be 10mg if there are no risk factors: age>60; body weight <50kg; liver disease; cardiac failure; low albumin; known bleeding risk; taking drugs that affect warfarin; previously anticoagulated at maintenance dose <2mg)
      • Subsequent doses are as follows (refer to local guidelines)
      • Picture1
    • Slow Initiation
      • Once initial checks (e.g. contraindications/bloodwork) has been carried out, start on a low dose of warfarin (e.g. 2mg/day; 1mg if patient is unwell/potential for drug interactions) for a week.
      • Check INR after this and adjust dose as required:
      • Picture1
      • In general, if the patient is taking an antiplatelet, this can be continued until the INR is in therapeutic range and then stop.  NB patients who have had an ACS in the past 12 months or have got a stent may require dual anticoagulant/antiplatelet therapy
  •  Explain monitoring/dose adjustments (once INR therapeutic and maintenance dose reached)
    • If INR is abnormally high or low (i.e. sub- or supra-therapeutic)- consider a change in dose (see below) and recall in 7-14 days
    • In general, if the INR is within therapeutic range, the patient can be monitored by increasing weekly intervals (i.e. 1 normal measurement- 1 week; 2 normal measurements- 2 weeks etc).
      • More than 5 therapeutic INRs suggest stable therapy and patients can be seen every 8-12 weeks following this
    • If the patient becomes unwell or there is any other reason to increase the frequency of monitoring- ensure the patient understands the importance of notifying the doctor/clinic.
    • Dose adjustments
      • For target INR 2-3 (2.5)
        • If low-
          • increase dose by 10% of total weekly dose (e.g. if the patient takes 21mg/week- increase the dose by 2mg/week) if the INR is consistently slightly low (1.8-1.9) or if the INR is low (1.6-1.8) at any time
            • This may mean alternating daily doses.
            • Also review INR in ~2 weeks
          • consider a loading dose (150% of maintenance dose) and increasing dose if the INR is lower than 1.6 (e.g. if the patient normally takes 6mg/day- consider a one-off dose of 9mg and increasing the dose by 4-5mg/week)
            • review INR in several days/one week
        • If high
          • decrease dose if the INR is consistently slightly high (3-3.2) or if the INR is ever high (3.4-3.9)
            • Review INR in ~2 weeks depending on severity
          • if the INR is significantly high (4-5), consider omitting one dose and reducing overall dose.  If >5, it may be appropriate to omit >1 dose.
            • Review INR in a few days
          • If the INR >6, stop warfarin until INR <5 and consider vitamin K reversal
            • Review INR next day if the patient is stable and not bleeding (oral vit K (1mg) is suitable)
            • If the patient is bleeding, admit for IV vitamin K (usually 1mg if not compromised; 5mg if in haemodynamic compromise +/- clotting factors/prothrombin (or FFP if not available))

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