Warfarin
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Warfarin resistance and subtherapeutic INR following drug-food interaction: case report A 33-year-old man exhibited warfarin resistance and subtherapeutic INR following concomitant administration of warfarin as an anticoagulant therapy and smokeless tobacco. The man had a history of unspecified IV drug use complicated by infective endocarditis, which required a mechanical mitral valve placement. He had been receiving oral warfarin by mouth daily [initial dose not stated] with a goal INR of 2.5 to 3.5. In February 2015, he stopped opioid usage after the valve replacement. He admitted chewing tobacco daily for the past 16 years. In Oct 2019, he presented for detoxification after a 4-week history of heavy drinking. During these 4-weeks, he had stopped taking warfarin. His INR upon admission was 0.9. An IV heparin drip was started to bridge him back to warfarin. His most recent warfarin dose was 15mg daily. He remained hospitalised between 10–24 October 2019 due to failure to achieve therapeutic INR despite warfarin doses of up to 30mg daily. After careful consideration, causes of subtherapeutic INR, including nonadherence to drug, dietary causes and genetic factors were ruled out. Therefore, the cause of warfarin resistance was considered to be an intake of a high amount of exogenous vitamin K from his chronic and heavy usage of smokeless tobacco [duration of treatment to reaction onset not stated]. The chewing of smokeless tobacco was continued during the hospital stay. On 24 Oct 2019, the man was administered three doses of warfarin 30mg, which increased his INR to 2.4. Heparin was discontinued, and enoxaparin-sodium was started. He was discharged on warfarin 20mg daily and enoxaparin-sodium. He was told about the interaction between warfarin and smokeless tobacco, and advised to discontinue the tobacco. However, he adamantly refused any attempt to stop chewing tobacco. Therefore, he was continued at an increased dose of warfarin to maintain the target INR. Nicolas D, et al. Subtherapeutic INR due to warfarin interaction with smokeless tobacco. Journal of Thrombosis and Haemostasis 18: 2954-2957, No. 11, Nov 2020. Available 803519526 from: URL: http://doi.org/10.1111/jth.15057
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Reactions 5 Dec 2020 No. 1833
