Published sources on warfarin interactions with other drugs, food, and supplements tend not to agree
Research Activities, May 2010, No. 357
Physicians and pharmacists rely on common drug compendia such as Clinical Pharmacology, ePocrates®, and Micromedex to head off possible adverse events for patients who take the blood thinner warfarin. However, a new study finds that these compendia and the drug label for warfarin often do not agree on which medications, supplements, or food can cause patients to experience dangerous interactions.
Researchers at the University of Arizona Center for Educational Research and Therapeutics (CERT) at the Critical Path Institute found that ePocrates® listed 182 entries for drug interactions with warfarin, while Clinical Pharmacology listed 201 and Micromedex listed 427. In fact, the three compendia agreed on only 50 items that could potentially interact with warfarin: 47 were drugs, 1 was a biologic (the influenza vaccine), and the other 2 were vitamin K and ethanol. The three compendia also differed in the terminology used to describe the potential interaction with warfarin. One used severity ratings ranging from "very high severity" to "low severity," while another used a scale of "major," "moderate," and "minor."
The authors suggest that this study shows the challenges clinicians face in determining drug and food interactions with warfarin. Prescribers must sort through vast amounts of information, which at times can be inconsistent or vague, to determine if a potential interaction is clinically significant. The authors recommend an authoritative body take on the tasks of creating consistent terms for compendia to use. This body should also conduct a thorough literature search to evaluate the evidence base to determine the list of drugs, supplements, and foods that interact with warfarin. The study was funded in part by the Agency for Healthcare Research and Quality (HS17001).
See "Warfarin interactions with substances listed in drug information compendia and in the FDA-approved label for warfarin sodium," by Marietta Anthony, Ph.D., Klaus Romero, M.D., M.S., Daniel C. Malone, Ph.D., and others in Clinical Pharmacology & Therapeutics 86(4), pp. 425-429, 2009.