Possible warfarin interaction with menthol cough drops.

Objective: To report a case of possible interaction of menthol cough drops (Halls) with warfarin in a patient awaiting cardioversion.

Case Summary: A 57-year-old white male awaiting cardioversion for atrial fibrillation was prescribed warfarin. His dosage was adjusted to 7 mg daily to provide stable international normalized ratio (INR) values of 2.28-2.68. Approximately one week later, his INR fell to 1.45. During a follow-up interview, the patient reported that he experienced a flu-like illness during the previous week and had been using menthol cough drops. No other potential causes for the decreased INR were found. Illness will most often elevate the INR; we therefore concluded that the cough drops were the likely cause of this reaction, and the warfarin dose was increased to 53 mg/wk. After discontinuing use of menthol cough drops, the warfarin dose was returned to the previous amount and the INR remained stabilized.

Discussion: An objective causality assessment suggests that the decreased INR was possibly related to the use of menthol cough drops during warfarin therapy. The active ingredient in these cough drops is menthol. Menthol has been shown to affect the pharmacokinetics of other drugs by inducing or inhibiting cytochrome P450 isoenzymes and slowing drug absorption. It is not clear whether these mechanisms played a role in this case. As of January 5, 2005 this is the first case report documenting an interaction between warfarin and cough drops containing menthol.

Conclusions: This case documents a significant decrease in the INR following the use of menthol cough drops. Patients who are ill have several factors that can potentially affect their INR and should be monitored closely.