Ramsay et al. reported from a United Kingdom survey that 92% of patients admitted to taking herbal medicines while receiving warfarin, noting that significant numbers of patients are in need of close monitoring. They also note that other CAM can interact with warfarin to increase bleeding time or act as anti-platelet agents (e.g., high dose vitamin E [> 400 IU], fish oils, garlic, St. John’s Wort, etc.) They particularly note that all patients receiving warfarin or that will be commencing warfarin be specifically asked about their use of CAM. The mechanism for this interaction still is unclear, but may be due to the inhibition of warfarin metabolism by the CYP2C9 or CYP3A4 enzymes, or a pharmacodynamic interaction between glucosamine and warfarin.
It is critical all patients, particularly those taking warfarin, have a full drug history at all hospitalizations. Make sure you inquire about vitamins and other supplements/CAM. If someone else is doing the intake medication history, make sure they get this information. A nurse might not think to ask. TH
Michele B. Kaufman, PharmD, BSc, RPh, is a registered pharmacist based in New York City.
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