I. Drugs that have synergistic effects with Warfarin 1. The combination of fluoroquinolones and Warfarin can cause coagulation disorders The Canadian Adverse Drug Reaction Bulletin (2004, Issue 3) states that cases of increased anticoagulant activity due to the combination of fluoroquinolone antibiotics and Warfarin have been reported repeatedly. The anticoagulant mechanisms of the interaction include: substitution of warfarin from protein binding sites; reduction of the intestinal flora that produces vitamin K and coagulation factors and decreased metabolism of warfarin. Most fluoroquinolone antibiotics inhibit cytochrome P450-mediated metabolism and therefore have the potential to cause toxic reactions to other drugs applied concomitantly, especially those with a narrow therapeutic index, such as warfarin, by decreasing drug clearance. Therefore, it is believed that fluoroquinolone antibiotics may enhance the effect of warfarin or its derivatives. Close monitoring of prothrombin time and INR of patients, especially in elderly patients, is welcome, and the dosage of warfarin should be adjusted at any time. 2, macrolides combined with Warfarin have an increased risk of bleeding The Australian Adverse Drug Reaction Bulletin reported reports of interactions between Warfarin and all four macrolide antibiotics (azithromycin, clarithromycin, erythromycin and roxithromycin). In contrast to this report, McCall et al, Texas Tech University Health Sciences Center, USA, reported that no significant change in INR was found after the addition of azithromycin in those taking Warfarin. There are also case reports of elevated INR and hematuria caused by amoxicillin. 3. Tramadol has synergistic effects with warfarin The Australian Adverse Drug Reaction Bulletin stated that tramadol caused an increase in INR and bleeding events in patients taking warfarin. Patients on a stable dose of warfarin had an onset 3-7 days (median time 4 days) after the addition of tramadol. Recovery occurred within 1-4 days after discontinuation of tramadol (no change in warfarin dose). Boeijinga et al. conducted a pharmacodynamic study and the results suggested that anticoagulation was associated with metabolic variability in a small number of patients. The Australian Council on Adverse Drug Reactions advises physicians that INR should be monitored for the first few days to a week after the addition of tramadol in patients treated with warfarin to prevent untoward events. Interferon has also been reported to increase the anticoagulant effect of warfarin. 4, fish oil and Warfarin have interaction Fish oil is an omega-3 polyunsaturated fatty acid, which can affect platelet aggregation and/or vitamin K-dependent coagulation factors, reduce the content of thromboxane A2 in platelets, and also reduce the level of factor VII, which can enhance the anticoagulant effect of Warfarin after proper supplementation. 5, antiphospholipid antibodies can cause an increase in INR Ashley et al. of the University of California Davis Medical Center reported that the investigators noted that INR values are often used to evaluate the degree of anticoagulation, and that an increase in INR represents a decrease in the activity of coagulation factors II, VII, and X. In rare cases, the presence of non-vitamin K-dependent inhibitors or interfering substances can interfere with the INR determination. In such cases, the anticoagulant effect of warfarin can be evaluated by direct measurement of coagulation factor concentrations. 6. Genetic variants increase the risk of bleeding during anticoagulation with warfarin A study at the University of Washington showed that variants in two genes related to warfarin metabolism increased the risk of bleeding during anticoagulation with warfarin. A randomized, double-blind, placebo-controlled clinical trial by Yuan et al. at the University of Chicago showed that American ginseng reduced the anticoagulant effect of warfarin. The results showed that the INR of the American ginseng group was significantly lower than that of the placebo group after 2 weeks. Therefore, it is recommended that physicians should inquire about the use of American ginseng in detail before administering warfarin to patients. 2. Ribavirin Schulman of Sweden reported a case of a 61-year-old male patient who had undergone heart valve replacement and was treated with warfarin for a long time. The mechanism of action is not known. Weekly monitoring of prothrombin time is recommended during the use of ribavirin. Warfarin has a narrow therapeutic window, and a minor effect on anticoagulation can have significant consequences.