Chitosan potentiation of warfarin effect.

OBJECTIVE

To report a case in which the anticoagulation effect of warfarin appeared to have been potentiated by chitosan, probably due to interference with the absorption of vitamin K.

CASE SUMMARY: An 83-year-old male with hypertensive cardiovascular disease, type 2 diabetes mellitus, and chronic atrial fibrillation complicated by left atrial thrombus formation was maintained on warfarin 2.5 mg/day. Marked elevation of the international normalized ratio (INR) was noticed after self-medication with chitosan 1200 mg twice daily. He denied taking any other drugs, natural substances, herbal medicines, and nutritional supplements, and stated that he had not changed his dietary habits. After parenteral administration of vitamin K and discontinuation of chitosan, the INR returned to within the target range. However, the patient took chitosan again, and the INR increased to well above the target range. Following strong medical advice, the patient stopped taking chitosan, and the INR remained stable thereafter.

DISCUSSION: Chitosan is a positively charged polymer that binds to the negatively charged lipids and bile acids in the gastrointestinal tract. It can affect the absorption of vitamins A, D, E, and K. Therefore, the anticoagulation effect of warfarin may be potentiated by chitosan through this mechanism. Use of the Naranjo probability scale revealed that the adverse effect was probably due to chitosan.

CONCLUSIONS

The interaction between warfarin and chitosan has not previously been reported. Healthcare professionals should be aware of this potential interaction.