How Smoking Cessation Could Affect Warfarin Treatment

the MPR take:

A case report in the Journal of Pharmacy Practice is the first published instance of a patient that required >30% reduction in weekly warfarin dose after smoking cessation. The 51-year-old male was diagnosed with acute deep vein thrombosis (DVT) and warfarin treatment was initiated; the patient’s internalized normalized ratio (INR) was subtherapeutic at 1.9 and the INR was eventually stabilized at a weekly dose of 46mg. Three weeks after the patient reduced his smoking from one pack to half a pack of cigarettes per day, the INR level was modestly subtherapeutic at 3.4 and the weekly dose was reduced to 44mg. After INR fluctuation over the course of a month, the patient revealed that he had quit smoking entirely and the INR level was now 4. After peaking at 5.1, the INR level fell to 3.1 after the warfarin dose was reduced further to 28mg weekly. This reflects a 39% dose reduction after smoking cessation for the INR level to remain in therapeutic range. It is believed that the warfarin-smoking effect is due to the mediation of the metabolism of warfarin through various P450 hepatic enzymes and the potential interaction between polycyclic aromatic hydrocarbons in cigarette smoke and enzyme induction of the P450 enzyme CYP1A2 (smoking cessation would therefore reverse enzyme induction). Additional research is needed to further evaluate warfarin and smoking cessation, including pharmacokinetics.

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.

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