To investigate the impact of interacting drugs on the dispensed doses of 
warfarin in the Swedish population. This was a retrospective, cross-sectional 
population based register study of patients being dispensed warfarin. Warfarin 
doses were estimated in different age groups, in men and women, and in patients 
using interacting drugs. The influence of interacting drugs on the dispensed 
warfarin dose was analyzed using multiple regression. All 143,729 patients 
dispensed warfarin were analyzed. The dispensed dose of warfarin was highest in 
patients 30-39 years old and decreased with age. Co-medication with 
carbamazepine, simvastatin, paracetamol, amiodarone, fluconazole, lactulose, or 
bezafibrate was associated with significant changes in dispensed warfarin doses, 
by +40%, -3.4%, -7.3%, -8.2%, -8.8%, -9.0%, and -9.7%, respectively. After 
adjustment for age and gender, sulfamethoxazole was also found to significantly 
alter the dispensed warfarin dose (-6.1%). We provide new support for the 
previous scarce evidence of interactions between warfarin and carbamazepine, 
bezafibrate, and lactulose. Initiation or discontinuation of bezafibrate or 
lactulose in a patient on warfarin should warrant close clinical monitoring. The 
marked increased warfarin requirement associated with carbamazepine use supports 
moving from a more conservative reactive towards a proactive strategy including 
preventive warfarin dose adjustments to avoid potential adverse effects.