Dietary supplements, herbs and oral anticoagulants: the nature of the evidence

In the US, the use of dietary supplements, including vitamins, minerals, amino acids, and herbal products, is extensive. Nonetheless, the majority of patients report that they have little information about the risks, benefits, and adverse effects of medicines, or about their potential interactions with prescription drugs. Patients taking warfarin are at particular risk of interactions with dietary supplements, yet approximately 30% use herbal or natural product supplements on a regular basis. No current governmental regulations or voluntary programs address dietary supplement interactions with prescription drugs. Case reports represent the majority of the evidence surrounding drug interactions between warfarin and dietary supplements. Those of the highest quality include, as an assessment of causality, a modification of the recently published Drug Interaction Probability Scale. Despite positive case reports, formal drug interaction studies are often negative, suggesting that numerous patient-specific influences other than the suspected interaction alone may be responsible for a particular observation. The cranberry-juice/warfarin interaction is a recent example of such a discrepancy. Healthcare providers can play an active role in improving quantity and the quality of case reports of interactions involving warfarin and dietary supplements. A registry of anticoagulant interactions with dietary supplements has been proposed, and is currently being developed through Clotcare Online Resource (http://www.clotcare.com). The goal of this registry is to obtain high quality case-based evidence of drug interactions between anticoagulants and dietary supplements, to define these interactions based on clinical and monitoring outcomes, and to analyze likelihood of causation using a modification of the Drug Interaction Probability Scale.