Predictable reduction in anticoagulant activity of warfarin by small amounts of vitamin K.

After withdrawal of warfarin (Marevan), 48-72 hours are required to raise the Thombotest values from a therapeutic level (range 5-10%) to 12% or higher. By i.v. administration of 1 mg vitamin K1 (Konakion) and without changing the dose of the anticoagulant, this effect could be obtained within 24 hours. Furthermore, the effect of vitamin K vanished within 2-5 days. The use of small amounts of vitamin K may therefore be a simple and reliable way of obtaining a rapid, temporary reduction in anticoagulant effect.