Warfarin, a drug with a narrow therapeutic index, has the potential to interact with many herbal remedies. For example, warfarin should not be combined with any of the following herbs because they may have additive or synergistic activity and increase the risk for bleeding: celery, chamomile, clove, dong quai, feverfew, garlic, ginger, ginkgo biloba, ginseng, green tea, onion, passion flower, red clover, St. John's wort, and tumeric. Any herbal remedy should be used with caution in patients taking warfarin.
Much of the information on drug-herb interactions is speculative. Herb-drug interactions are sporadically reported and difficult to determine. Because herbal supplements are not regulated by the Food and Drug Administration (FDA), products lack standardization, purity, and potency. In addition, multiple ingredients in products and batch-to-batch variation make it difficult to determine if reactions occur as the result of the herb. To assist with the identification of herb-drug interactions, nurses should report any potential interactions to the FDA through its MedWatch program (see Appendix A). Because the absorption, metabolism, distribution, and elimination characteristics of most herbal products are poorly understood, many herb-drug interactions are speculative. It is especially important to take special care when patients are taking any drugs with a narrow therapeutic index (the difference between the minimum therapeutic and minimum toxic drug concentrations is small-such as warfarin) and herbal supplements.
use of any drug, both prescription and over-the-counter preparations.
• The Patient Receiving Warfarin ASSESSMENT
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