| Warfarin vitamin interactions Vitamin A Possible Harmful Interaction Supplemental vitamin A might increase the blood-thinning effects of warfarin, and this could potentially lead to an increased risk of abnormal bleeding. Vitamin C Possible Harmful Interaction Vitamin C taken in high dosages (more than 1,000 mg daily) has been reported to reduce the blood-thinning effect of warfarin. In one case, the person was taking 1,000 mg of vitamin C daily; another involved huge mega doses (about 16,000 mg daily). Vitamin E Possible Harmful Interaction On the basis that vitamin E "thins" the blood, it has been suggested not to combine vitamin E with warfarin. However, a 4-week double-blind study of 25 individuals taking warfarin found no additive effect. None of the participants taking vitamin E at a daily dose of 800 or 1,200 IU (international units) showed an increased risk for abnormal bleeding. In contrast, a case report indicated that vitamin E (800 IU daily) added to the effects of warfarin and resulted in abnormal bleeding. Because this effect did not become apparent until the fourth week, it is possible that problems might take longer to develop than the 4-week period covered by the double-blind study, or that certain individuals might be more prone to an interaction. An unpublished 30-day study of three volunteers taking a warfarin-like drug also found an additive effect with only 42 IU of vitamin E daily. Vitamin K Possible Harmful Interaction Vitamin K is an antidote to warfarin—it directly counteracts warfarin's blood-thinning effects. This is true for both supplemental vitamin K and foods high in vitamin K. For this reason, eating more vitamin K–rich vegetables can decrease warfarin's therapeutic effect, and eating less of these foods can increase the drug's effect. Either situation can lead to potential life-threatening complications. |
Drug Interactions
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