Menadione is a metabolite of oral vitamin K.

Phylloquinone is converted into menaquinone-4 and accumulates in extrahepatic tissues. Neither the route nor the function of the conversion is known. One possible metabolic route might be the release of menadione from phylloquinone by catabolic activity.

Paracetamol (acetaminophen) warfarin interaction: NAPQI, the toxic metabolite of paracetamol, is an inhibitor of enzymes in the vitamin K cycle.

Paracetamol (acetaminophen) is generally considered to be the analgesic of choice for patients undergoing oral anticoagulant therapy. Occasionally, however, interactions have been reported with therapeutic doses of the analgesic, e.g.