Warfarin

Dosage Forms

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Tablet

1 mg

5 mg

Uses

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Deep vein thrombosis
Pulmonary embolism

Dose and Duration

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All uses

Adult: induction dose 10 mg once daily for 2 days adjusted to meet individual requirements. Daily maintenance dose is usually 3 to 9 mg taken at the same time each day, determined based on the prothrombin time.

Child 1 month–18 years: 0.2 mg/kg (max 10 mg) as a single dose on first day, reduced to 0.1 mg/kg (max 5 mg) once daily for following 3 days. If INR is still below 1.4, use 0.2 mg/kg (max 10 mg) once daily. If INR is above 3 use 0.05 mg/kg (max 2.5 mg) once daily. If INR is above 3.5, omit dose, then adjust according to INR, usual maintenance 0.1–0.3 mg/kg once daily.

Neonate: specialist use only.

Contra-indications

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Haemorrhagic stroke
Within 48 hours post–partum
Clinically significant active bleeding
Within 72 hours of major surgery with risk of severe bleeding

Side Effects

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Haemorrhage
Nausea, vomiting, diarrhoea, melaena, vomiting blood
Jaundice, hepatic dysfunction
Alopecia
Fever
Hypersensitivity, rash

Interactions

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Acetylsalicylic acid, diclofenac (increased risk of bleeding)
Alcohol (enhanced anticoagulant effect)
Amitriptyline, corticosteroids, nevirapine (enhanced or reduced anticoagulant effect)
Azathioprine, mercaptopurine (anticoagulant effect possibly reduced)
Carbamazepine, griseofulvin, phenobarbital, rifampicin, phenytoin (reduced anticoagulant effect)
Azithromycin, cefazolin, cefixime, ceftazidime, ceftriaxone, doxycycline, fluorouracil, fluoxetine, ibuprofen, levamisole (enhanced anticoagulant effect)
Glibenclamide (possibly enhanced hypoglycaemic effect and changes to anticoagulant effect)
Levonorgestrel, medroxyprogesterone, norethisterone, phytomenadione (antagonism of anticoagulant effect)
Ritonavir (concentration possibly increased by ritonavir)
Chloramphenicol, cimetidine, ciprofloxacin, clomipramine, clopidogrel, erythromycin, fluconazole, levothyroxine, metronidazole, miconazole, ofloxacin, silver sulfadiazine, simvastatin, sulfadoxine + pyrimethamine, sulfamethoxazole +trimethoprim, tamoxifen, tramadol, testosterone (enhanced anticoagulant effect)

Pregnancy

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Do not use in 1st and 3rd trimester.

Breast-feeding

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Can be used.

⚠️ Caution

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Effect takes 48–72 hours to develop fully. Use with heparin in the first 3 days to allow for warfarin effect to set in.
Females of child-bearing age on warfarin should use effective contraception during treatment.
Reduce induction dose in the elderly.
Avoid in severe hepatic failure especially if prothrombin time is already prolonged.
Use with caution in mild to moderate renal failure. Avoid use in severe renal failure.