Potentiation of Warfarin Anticoagulation Associated with Topical Methyl Salicylate

OBJECTIVE: To report a case of international normalized ratio (INR) elevation resulting from the administration of topical methyl salicylate in a patient whose INR was previously stable while she received warfarin anticoagulation. CASE SUMMARY: A 22-year-old white woman presented with an INR of 12.2 after applying a topical pain-relieving gel to her knees daily for eight days. The potentiation of the warfarin anticoagulation was attributed to the low-dose methyl salicylate contained in the product. DISCUSSION: Methyl salicylate is systemically absorbed through the skin in measurable amounts, and may increase warfarin action by affecting vitamin K metabolism or by displacing warfarin from protein-binding sites. While several investigators have reported this interaction with use of high-dose methyl salicylate, this case indicates that a significant interaction can occur with the use of lower topical doses of methyl salicylate as well. CONCLUSIONS: Healthcare providers and patients taking warfarin must be aware of the potential hazard of using topical methyl salicylate in combination with warfarin.