A composition is provided that, when utilized in combination with a coumarin anticoagulant, greatly improves the anticoagulant effects of the anticoagulant 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 n is 1 or 2, where R1 and R4 are hydrogen or acyl, R2 is a hydrogen or saturated or unsaturated alkyl group with up to 6 carbons, and R3 is a hydrogen or saturated or unsaturated alkyl group with up to 6 carbons, or R2 and R3 are part of a cyclic or polycyclic ring system.
