Short answer: The evidence is mixed. NCCIH calls it conflicting. Case reports linked cranberry juice to higher INR on warfarin, but two small controlled trials found no meaningful change with about a cup a day for one to two weeks. A 2021 review still listed cranberry among foods that may raise bleeding risk. The warfarin label says to check INR more often when starting or stopping herbal products.
A 2007 review (Pham 2007) traced the concern to reports from the United Kingdom of people on warfarin whose INR rose after drinking cranberry juice. It found three published case reports. Two suggested a link but neither clearly showed cranberry was the only cause. The proposed mechanism was that cranberry flavonoids might slow CYP2C9, the main enzyme that clears warfarin, but two controlled trials of other drugs cleared by the same enzymes found no effect.
| Study | Design | Result |
|---|---|---|
| Li 2006 | 7 men with atrial fibrillation; 250 mL a day for 7 days vs placebo, crossover | No significant INR change at any time point |
| Ansell 2009 | 30 people; 240 mL a day for 2 weeks vs matched placebo drink | No change in warfarin levels; INR differed on 1 day only, judged unlikely to matter |
Ansell 2009 noted that minimally raised INR values happened in 4 people on cranberry and 4 on placebo, and suggested earlier case reports may have been chance timing. Li 2006 said daily juice at 250 mL did not seem to need cutting out, but that INR should be followed closely.
Both trials were small and short and used ordinary juice amounts. They do not test concentrated cranberry capsules or very large amounts.
NCCIH says "there is conflicting evidence about whether cranberry interacts with the anticoagulant (blood thinner) warfarin," and advises people on warfarin to talk with a provider before using cranberry products. A 2021 systematic review of food and supplement interactions with warfarin (Tan 2021) listed cranberry among items healthcare professionals should know may raise bleeding risk, alongside ginkgo, green tea and St. John's wort.
The warfarin label (section 7.5) does not name cranberry, but says few good studies exist for botanicals and that INR should be checked more often when starting or stopping them, because product strength varies.
The theme across warfarin interactions is change: starting, stopping or sharply changing a food or supplement is what shifts INR. The same idea applies to vitamin K foods; see vitamin K and warfarin. Dose Track can log a new supplement as a note on the day it starts, so it lines up with the next INR check.
Read more on Dose Track: Dietary supplement · CYP3A4 · Interaction checker in Dose Track