Warfarin resistance can be defined as the inability to prolong the international normalized ratio (INR) into the therapeutic range when the anticoagulant is given at normally prescribed doses [1]. Warfarin inhibition by abusive green tea intake was ascribed to green tea high vitamin K content [2]. However, when infusions of green tea were analyzed, the concentration of vitamin K was lower than the detection limit [4]. At the Anticoagulation Clinic of Hospital dos Servidores do Estado, Rio de Janeiro, Brazil, three patients could overcome warfarin resistance when caffeine intake was restricted. It is important to notice that coffee and cola beverages also have low vitamin K content [3]. A 27-year-old female with previous history of four fetal losses and a stroke, had a diagnosis of antiphospholipid syndrome confirmed, and was placed on warfarin. Nonetheless, with 20 mg/day her INR repeatedly remained below 1.3. She was drinking more than 2 l of a Brazilian caffeine-rich soft drink guarana per day, six days a week. After guarana was withhold, her INR reached target range with 7.5 mg/day. She started to drink guarana again and her INR dropped to 1.2 after five days. An 18-year-old male had a 3rd episode of deep venous thrombosis when antiphospholipid syndrome diagnosis was made. Anticoagula-tion was started, but with warfarin 20 mg/day his INR remained below 1.5. He used to take phenobarbital to control epilepsy and to drink about 1.5 l of regular cola per day. When daily cola intake was limited to 350 ml, INR stabilized between 2.5 and 3 with warfarin 7.5 mg/day, still on phenobarbital. After being within or above the target range for several years, during a regular check up his INR was found to be 1.1. He referred a regular coffee intake of more than five espressos per day. Coffee was restricted to one espresso per day and his INR stabilized within therapeutic range. A 27-year-old HIV positive male with low free protein S levels, was placed on anticoagulation due to a deep venous thrombosis and a giant maleolar ulcer. He was an abusive coffee drinker, unable to attain INR target range with warfarin 20 mg/day. When coffee was limited to one cup per day, INR increased to 8 in four days, and stabilized within therapeutic range with 5 mg/day. Warfarin occurs as a pair of enantiomers: R-warfarin is metabo-lized primarily by cytochromes P450 (CYP) 1A2, 3A4, and by carbonyl reductases, and S-warfarin is metabolized primarily by CYP2C9. Because S-warfarin has higher potency than R-warfarin the efficacy of warfarin is affected primarily when metabolism of S-warfarin is altered [5]. Exposure to caffeine induces the expression of the enzyme CYP1A2 [6], but at a higher caffeine concentration, the contribution of CYP1A2 to caffeine metabolism decreases in favor of CYP2C9 [7], a situation that could affect S-warfarin metabolism. Additionally, CYP450 polymorphisms [8] could explain variability of responses to caffeine in warfarin users. We recommend that coffee and other caffeine-containing bever-ages intake to be assessed in medical history of warfarin users and that caffeine restriction be tested as a way to overcome warfarin resistance.