Phytomenadione (Vitamin K1)

How it works

It acts as an essential cofactor for hepatic gamma-carboxylation of clotting factors II, VII, IX and X, restoring synthesis of functional vitamin K-dependent coagulation factors.

Prescribing in practice

• Intravenous administration can rarely cause anaphylactoid reactions, so it should be given slowly and, where the clinical situation allows, the oral or another route considered.
• When reversing warfarin, the dose and route depend on the degree of over-anticoagulation and whether bleeding is present; excessive correction can make re-anticoagulation difficult.
• It is routinely offered to newborns to prevent vitamin K deficiency bleeding; paediatric use should follow a children's formulary.

Monitoring

Monitor the INR and clinical response after administration, as the full effect on the INR may take several hours.

Counselling the patient

• Explain to parents that vitamin K is given to newborns to prevent a rare but serious bleeding disorder.
• Advise patients on warfarin that vitamin K reverses their anticoagulation and that further INR checks will be needed.

Evidence & guidelines

Guidance on warfarin reversal and neonatal vitamin K prophylaxis is well established in UK haematology and paediatric practice.