BACKGROUND: A potential drug interaction exists between oral corticosteroids and 
warfarin, but there is limited documentation.
OBJECTIVE: To evaluate the potential drug interaction between oral 
corticosteroids and long-term warfarin therapy.
METHODS: A retrospective review was conducted of 387 medical records for active 
patients within an anticoagulation clinic. Inclusion criteria were stable 
anticoagulation therapy, short-term oral corticosteroid therapy, international 
normalized ratio (INR) recorded within 30 days prior to corticosteroid 
initiation (pre-INR), and INR recorded during corticosteroid therapy or within 
14 days of discontinuation (post-INR). Patients were excluded if they had been 
started on any antibiotic or other drug with a probable interaction with 
warfarin at the same time as corticosteroid initiation. Thirty-two patient 
encounters met the predetermined inclusion and exclusion criteria. The primary 
outcome assessed was the difference between pre- and post-INR values. Secondary 
endpoints included bleeding events, emergency department (ED) visits, 
hospitalizations, and warfarin dose modifications.
RESULTS: The mean difference between pre- and post-INR values was 1.24 (95% CI 
0.86 to 1.62). Ninety-seven percent of the 32 patient encounters resulted in a 
change in their post-INR value, and 62.5% of patients had supratherapeutic INR 
values at the post-corticosteroid assessment. The majority of patients assessed 
had an elevation of their INR following concomitant use of warfarin and 
corticosteroids. The INR change was observed at a mean +/- SD of 6.7 +/- 3.3 
days following the first dose of corticosteroid. Overall, 16 patients (50%) 
required a modification of their anticoagulation therapy during or following 
corticosteroid therapy. Only one adverse event of minor epistaxis was reported, 
and no ED visits or hospitalizations occurred as a consequence of the drug 
combination.
CONCLUSIONS: Use of oral corticosteroids in patients on long-term warfarin 
therapy may result in a clinically significant interaction, which requires close 
INR monitoring and possible warfarin dose reduction.