The potential risk of herb drug interactions is of particular focus today owing to the increasing and inadvertent use of herbs in recent times. It is a major safety concern for the drugs with narrow therapeutic index like warfarin, a most common anticoagulant with the maximum number of interactions reported. The objective of the present study was to conduct a systemic review of literature to consolidate the clinical case reports of warfarin–herb interactions and to assess the report reliabilities. We reviewed the published clinical literature to consolidate and
assess the interactions between various herbs and warfarin, based on reported adverse events, descriptions of the clinical case reports and case series using electronic databases as well as hand picked references from the year 1971 to year 2007 and ranked them on likely causality
using Naranjo’s algorithm. Out of 72 cases of documented case reports of warfarin with various herbs, 84.7% cases were evaluated as possible interactions (61/72) and 15.3% cases (11/72) as probable interactions. Cranberry juice was most commonly involved in interactions with warfarin with 34.7% of cases (25/72) of which 92% cases were possible interactions (23/25) and 8% cases (2/25) were probable
interactions. Hence, we conclude that combining anticoagulant medicines with herbs appears to be a risky proposition. The number of herbs reported to interact with warfarin continues to expand. Patients on warfarin are specifically advised to avoid taking herbal medicines or to
have their INR measured within two weeks of starting the drug, to be on a safer side. Further, more systematic studies pertaining to warfarin herb interactions are urgently warranted.