Patient case: impact of smoking cessation on international normalized ratio

The concurrent use of cigarettes while on warfarin therapy is a common occurrence. Smoking cessation among patients on chronic warfarin therapy is suspected to reduce drug clearance that may require dose adjustments. This type of interaction is particularly important when dealing with narrow therapeutic medications, as is the case with warfarin. Our case describes a series of supratherapeutic international normalized ratios (INRs) due to smoking cessation while on concomitant warfarin therapy. Patient Case: A 51-year-old male presented to the anticoagulation clinic for management of his warfarin therapy for an acute deep vein thrombosis. After 2 months of stable, therapeutic INR levels, the patient abruptly decreased his smoking from 1 pack/day to one-half pack/day and then subsequently stopped smoking completely. The patient's smoking cessation resulted in a major modification of his required weekly warfarin dose to maintain a therapeutic INR (a 39% dose reduction).

This case exemplifies how certain lifestyle factors, such as smoking, can alter the pharmacokinetics of patients on chronic warfarin therapy. This is the first case to demonstrate a greater than 30% reduction in the weekly warfarin dose following smoking cessation.