Kangen-Karyu (KGK), containing six herbs, is a formula created under the theory of traditional Chinese medicine (TCM) to invigorate the ‘blood’ and dispel ‘blood stasis’, which arises from poor blood circulation. This study was conducted to evaluate the interactions between KGK and warfarin. Warfarin was administered orally or intravenously to KGK-treated rats, and plasma warfarin concentration and prothrombin times were measured. Although KGK did not influence the absorption or serum protein binding of warfarin, it significantly suppressed the metabolism and elimination of warfarin. This interaction depends on the dosage of KGK, and 10 times the amount of the human daily dose of KGK did not exhibit pharmacokinetic interaction with warfarin, suggesting that KGK did not influence the effect of warfarin unless the daily dose was strictly maintained. Warfarin alone significantly prolonged mouse-tail bleeding time, which was further prolonged significantly by KGK at a dose that did not exhibit pharmacokinetic interactions with warfarin.