Vitamin C Interaction with Warfarin
Vitamin C does not significantly interact with warfarin and can be safely taken by patients on warfarin therapy without dose adjustments or additional monitoring.
Mechanism and Effects
- Vitamin C is not listed among substances that increase or decrease PT/INR response in the FDA warfarin drug label 1
- Unlike vitamin K, which directly antagonizes warfarin's action, vitamin C does not interfere with warfarin's anticoagulant mechanism 1
- High doses of vitamin C (not standard supplementation) may potentially decrease warfarin's effect according to the FDA label, but this is not considered clinically significant at normal supplementation doses 1
Clinical Considerations
- Patients on warfarin should focus on maintaining consistent vitamin K intake, as fluctuations in dietary vitamin K are a major source of warfarin variability 2, 3
- While monitoring vitamin K intake is essential, there is no reliable evidence indicating that vitamin C at standard supplementation doses affects warfarin anticoagulation 3
- Patients on vitamin K-restricted diets may inadvertently limit their vitamin C intake, as many foods rich in vitamin K (green vegetables) are also good sources of vitamin C 4
Monitoring Recommendations
- No special INR monitoring is required when initiating or discontinuing standard vitamin C supplementation 1
- Regular INR monitoring should continue as normally scheduled for patients on warfarin therapy 5
- If unusually high doses of vitamin C are being considered (significantly above RDA), more frequent INR monitoring may be prudent 1
Special Considerations
- In rare cases, vitamin C deficiency has been reported in anticoagulated patients on restricted diets, presenting as perifollicular hemorrhagic rash 4
- Patients should report all supplements they are taking, including multivitamins, as some multivitamin formulations contain small amounts of vitamin K that could potentially affect anticoagulation in susceptible patients 6
- Unlike antibiotics (such as fluoroquinolones, macrolides, and metronidazole) which significantly interact with warfarin, vitamin C supplementation is considered safe 7
Practical Advice for Patients
- Patients can safely take standard vitamin C supplements while on warfarin therapy 1, 3
- Maintaining consistent dietary habits regarding vitamin K intake remains the most important dietary consideration for warfarin stability 8, 2
- Patients should inform their healthcare provider about all supplements they are taking, even those perceived as harmless like vitamin C 6