Objectives: The interaction between amiodarone and
warfarin has only been described in patients being followed up for
relatively short time periods. The objectives of this study were to
characterize the interaction between these two agents in a clinical
situation over a longer period of time in a larger cohort of patients,
and to determine the relationship between the maintenance dose of
amiodarone and the resultant need to adjust the dose of warfarin.
Design: This was an observational trial of a cohort of
patients receiving a stable warfarin regimen in whom oral amiodarone
was initiated. Patients received both amiodarone and warfarin for at
least 1 year, and the dosage of warfarin was adjusted as clinically
necessary to achieve an international normalized ratio of 2 to 3. Data
from a total of 43 patients were analyzed.
baseline, prior to initiation of amiodarone, the warfarin dose was
5.2 ± 2.6 mg/d. The magnitude of the interaction between these two
agents peaked at 7 weeks, which resulted in a 44% mean maximum
reduction in the warfarin dose. The warfarin dose inversely correlated
with the maintenance dose of amiodarone
(r2 = 0.94, p < 0.005). Minor bleeding
episodes occurred in five patients (12%). For patients receiving
amiodarone maintenance doses of 400, 300, 200, or 100 mg/d, it is
recommended that the daily warfarin dose be reduced by approximately
40%, 35%, 30%, or 25%, respectively.
The magnitude of the amiodarone/warfarin interaction is highly
dependent on the maintenance dose of amiodarone. This relationship can
aid clinicians in adjusting the dose of warfarin patients receiving
long-term amiodarone treatment.