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Interaction Between Warfarin and Black Tea

TO THE EDITOR: Although it is known that many herbal preparations have the potential to interact with warfarin, to date, no interaction between warfarin and black tea has been documented. We describe such an interaction in a patient who drank black tea daily and was subsequently prescribed warfarin. Case Report. A 67-year-old white female was prescribed warfarin (target international normalized ratio [INR] 2–3) for atrial flutter and a history of a transient ischemic attack. After receiving warfarin at the same dosage for 3 months (32 mg/wk, with INRs ranging between 1.7 and 2.7), the patient stopped drinking black tea. She was unable to quantify the amount of black tea that she had been drinking each day and had reportedly alternated between drinking one brand of pure black tea (Red Rose Premium Blend Black Tea) and one brand of black tea that consisted of black tea, cloves, and natural orange flavor (Lipton Orange and Spice Black Tea). For each brand, the patient brewed tea bags in boiling water. Within one week after discontinuation of the black tea, the patient’s INR increased to 5.0. The weekly warfarin dose was subsequently decreased to 26 mg/wk (a 31% reduction), which remained unchanged for 2 months following the discontinuation of black tea; INRs during that time ranged between 1.7 and 3.3 (Figure 1). The patient denied any changes in prescription, over-the-counter, and herbal products. She also denied dietary changes, specifically, foods known to have a moderate-to-high content of vitamin K, grapefruit juice, and cranberry juice. In addition, there were no known changes in her health or medical condition. Use of Hansten’s Drug Interaction Probability Scale indicated a possible relationship between black tea and suppression of the INR in this patient.1 To support a stronger relationship (ie, a probable interaction) between the intake of black tea and INR suppression, this patient would have to be rechallenged with black tea or the amount of black tea intake would have to be increased or decreased, with INR monitoring. However, in light of her medical history and the clinician’s suspicion that an interaction had occurred, the patient was advised to avoid future consumption of black tea. Discussion. While an interaction between warfarin and green tea has been reported,2 an interaction between black tea and warfarin has not been established. Both black tea and green tea are derived from the dried leaves of Camellia sinensis, differing in their processing method in that green tea is made using unfermented leaves, while black tea is made using fully fermented leaves.3 Black tea, like green tea, may be a source of vitamin K1, thereby inhibiting the effects of warfarin.4 When consumed in sufficient quantities, black tea, like green tea, may suppress the INR. Additionally, black tea may have antiplatelet activity, and may increase