Pharmacology. Vitamin K1 is an essential cofactor in the hepatic synthesis of coagulation factors II, VII, IX, and X. In adequate doses, vitamin K1 reverses the inhibitory effects of coumarin and indanedione derivatives on the synthesis of these factors. Note: Vitamin K3 (menadione) is not effective in reversing excessive anticoagulation caused by these agents. After parenteral vitamin K1 administration, there is a 6- to 8-hour delay before vitamin K–dependent coagulation factors begin to achieve significant levels, and peak effects are not seen until 1–2 days after the initiation of therapy. The duration of effect is 5–10 days. The response to vitamin K1 is variable, and the optimal dosage regimen is unknown; it is influenced by the potency and amount of the ingested anticoagulant, vitamin K pharmacokinetics, and the patient's hepatic biosynthetic capability. Fresh frozen plasma or whole blood is indicated for immediate control of serious hemorrhage.