NEWCASTLE, UNITED KINGDOM. The evidence regarding a possible interaction between warfarin (Coumadin) and vitamin C is conflicting. A reduction in INR (shortening of bleeding time) was noted in 2 separate cases involving patients supplementing with 2 grams/day of vitamin C and an unspecified amount respectively. In contrast, a trial in which 5 patients supplemented with 1 gram/day for 14 days revealed no effect of vitamin C on INR. Another trial involving patients given 1 gram/day of vitamin C for 6 months demonstrated no change in required warfarin dosage compared with control patients not supplementing with vitamin C.
Finally, a trial involving 19 warfarin-treated patients given 3, 5, or 10 grams of vitamin C for 7 days showed no clinically important changes in INR, but did result in a 17.5% drop in total plasma warfarin concentration. The researchers involved in this study attribute the decreased absorption of warfarin to the loose stools or diarrhea often accompanying high vitamin C intakes.
A group of researchers at the University of Newcastle now report that normal dietary intakes of vitamin C (20-600 mg/day; 92 mg/day average) has no effect on warfarin clearance from the blood and thus is unlikely to affect INR. Their study involved 57 patients (31 males) who were receiving warfarin.
Wynne, H, et al. Dietary related plasma vitamin C concentration has no effect on anticoagulation response to warfarin. Thrombosis Research, Vol. 118, 2006, pp. 501-04
Editor's comment: The evidence regarding a possible effect of vitamin C on warfarin clearance and INR is clearly mixed. It would seem though that, while relatively small doses would be expected to have no effect, large doses (3-10 grams/day) may decrease plasma concentrations of warfarin and thus could potentially lead to a drop in INR. From the sparse data available, it would appear that the drop would be relatively minor (from 2.5 to 2.1, for example) and could safely be compensated for by a slight increase in warfarin dosage. In my opinion continuing supplementation (on a steady basis) with 3 x 500 mg/day of vitamin C when on warfarin would be far more beneficial than detrimental.