OBJECTIVES: To quantify and qualify the risk of major bleeding associated with the use of antibiotic drugs during coumarin anticoagulant therapy in daily practice. METHODS: Data for this retrospective cohort study were obtained from the PHARMO Record Linkage System, including, among others, linked drug-dispensing records and hospital records for over three million individuals in defined areas of the Netherlands. The study cohort included all users of acenocoumarol or phenprocoumon, aged 40–80 years, for the period 1996–2004. All patients were followed until the end of their last coumarin dispensing, hospitalisation for bleeding, death, or end of study period. Number of days on coumarins alone, and number of days on coumarins in combination with each antibiotic drug during follow-up, were determined for each patient. We analysed antibiotic drugs for which at least five patients experienced a bleeding at least two weeks after anticoagulant therapy commenced, and which fell within the risk period of a single antibiotic drug. RESULTS: Respectively, 52,102 acenocoumarol and 7,885 phenprocoumon users met the inclusion criteria for the study cohort and contributed 139,159 patient-years of follow-up. During follow-up 838 patients (1.4%) were hospitalised for bleeding while taking coumarins. The antibiotic drugs for which at least five coumarin users had a bleeding were doxycycline, amoxicillin, amoxicillin/clavulanic acid, ciprofloxacin and cotrimoxazole. Each of these antibiotic drugs was associated with an increased risk of bleeding. Incidence rates ranged from 5.2 major bleedings per 10,000 doxycycline-dispensings to 11.2 major bleedings per 10,000 cotrimoxazole-dispensings during coumarin use. Corresponding relative risks for major bleeding, adjusted for gender and age, ranged from 2.6 (95% CI 1.4–4.8) for doxycycline to 5.3 (95% CI 2.4–11.8) for cotrimoxazole. CONCLUSION: Doxycycline, amoxicillin, amoxicillin/clavulanic acid, ciprofloxacin and cotrimoxazole are the main antibiotic drugs associated with an increased risk of major bleeding during coumarin anticoagulant therapy in daily practice.