A composition is provided that, when utilized in combination with warfarin, greatly improves the anticoagulant effects of warfarin in mammalian subjects. The composition is a compound having a naphthohydroquinone ring system substantially similar to the ring system of the reduced form of vitamin K1 and has the general formula: where R1 and R4 are hydrogen or acyl, R2 is a saturated or unsaturated alkyl group with up to 6 carbons, and R3 is a saturated or unsaturated alkyl group with up to 20 carbons, or R2 and R3 are part of a cyclic or polycyclic ring system.