Opinions and research priorities

Probable Elevation in International Normalized Ratio from Cranberry Juice

Objective:

To report a case in which cranberry juice interacted with warfarin, resulting in an elevated international normalized ratio (INR).

Case Summary:

A 77-year-old male had maintained stable INR values for 6 months while on a warfarin maintenance dose of 24 mg/wk. On a routine follow-up examination, the INR was 5.9; one month prior to presentation, the INR had been 2.4. The patient reported several recent nosebleeds and a red mark on his left eye. He stated he had been adherent to therapy and denied any changes in medications or recent illnesses such as fever, flu, or diarrhea. His physical activity level and diet had been consistent, with no alcohol intake. The only change identified was the addition of daily cranberry juice (1–3 cups/day) for the last month. Cranberry juice was stopped, and the dose of warfarin was decreased to 22 mg/wk. Subsequently, the INR stabilized at the low end of the therapeutic range.

Discussion:

Whether cranberry juice interacts with warfarin is unclear. Three previously published case reports suggested a possible interaction. In at least 2 of these cases, confounding factors and incomplete information make assessing the influence of cranberry juice difficult. The interaction in our patient was scored as probable by the Naranjo probability scale, based on the temporal relationship, return to the therapeutic INR range after discontinuing cranberry juice, and elimination of alternative causes for the INR elevation.

Conclusions:

This case suggests that cranberry juice may interact with warfarin, leading to an elevated INR.

Enhanced Interaction between Warfarin and High-Dose Ketoconazole: A Case Report

This case describes the increased anticoagulation effect associated with the use of high-dose ketoconazole. A 59-year-old man treated with warfarin for aortic valve replacement was prescribed high-dose ketoconazole and hydrocortisone for the treatment of prostate cancer. Despite lowering the warfarin dosage by 35% during the start of high dose ketoconazole, an additional dose reduction was required subsequently when the INR rose from 2.62 to 3.82 within nine days. After a total dose reduction of 43%, the INR returned to therapeutic range within two weeks. The Naranjo probability scale revealed a probable adverse reaction of increased anticoagulant effect associated with high dose ketoconazole. Due to the inhibition of warfarin metabolism by ketoconazole, patients taking high dose ketoconazole concomitantly with warfarin may need their warfarin dosage reduced by more than is currently recommended, as well as receive more frequent INR monitoring to avoid over anticoagulation.

Interaction between warfarin and cranberry juice

INTRODUCTION

Warfarin is commonly used to treat or prevent thromboembolic events. Cranberry juice has been suggested to have an interaction with warfarin. However, there have been few reported cases of warfarin-cranberry juice interaction.

CASE SUMMARY

A 78-year-old, 86-kg man receiving warfarin at a total weekly dose of 45 mg for atrial fibrillation presented at the Bakersfield Healthcare Center of the VA Greater Los Angeles Healthcare System with an international normalized ratio (INR) of 6.45, having reported drinking a half gallon of cranberry/apple juice in the week prior to the elevated INR. After discontinuation of the cranberry juice, maintaining the warfarin dose for 5 days, and resuming the warfarin at a total weekly dose of 40 mg, the INR returned to the therapeutic range of 2 to 3.

DISCUSSION

Possible warfarin-cranberry juice interactions have been reported in the literature. Case reports illustrate INR elevation associated with cranberry juice ingestion concomitantly with warfarin administration and may be associated with bleeding (eg, pericardial, gastrointestinal). In the present case report, cranberry juice was the most likely cause of the patient's elevated INR. The Naranjo probability scale score was 3, suggesting that there was a possible interaction between warfarin and cranberry juice, while the modified Naranjo scale score adapted for anticoagulants was 5, rating the interaction as probable.

CONCLUSION

The combination of warfarin administration and cranberry juice ingestion appeared to be associated with an elevated INR without bleeding in this elderly patient.

Dietary supplements, herbs and oral anticoagulants: the nature of the evidence

In the US, the use of dietary supplements, including vitamins, minerals, amino acids, and herbal products, is extensive. Nonetheless, the majority of patients report that they have little information about the risks, benefits, and adverse effects of medicines, or about their potential interactions with prescription drugs. Patients taking warfarin are at particular risk of interactions with dietary supplements, yet approximately 30% use herbal or natural product supplements on a regular basis. No current governmental regulations or voluntary programs address dietary supplement interactions with prescription drugs. Case reports represent the majority of the evidence surrounding drug interactions between warfarin and dietary supplements. Those of the highest quality include, as an assessment of causality, a modification of the recently published Drug Interaction Probability Scale. Despite positive case reports, formal drug interaction studies are often negative, suggesting that numerous patient-specific influences other than the suspected interaction alone may be responsible for a particular observation. The cranberry-juice/warfarin interaction is a recent example of such a discrepancy. Healthcare providers can play an active role in improving quantity and the quality of case reports of interactions involving warfarin and dietary supplements. A registry of anticoagulant interactions with dietary supplements has been proposed, and is currently being developed through Clotcare Online Resource (http://www.clotcare.com). The goal of this registry is to obtain high quality case-based evidence of drug interactions between anticoagulants and dietary supplements, to define these interactions based on clinical and monitoring outcomes, and to analyze likelihood of causation using a modification of the Drug Interaction Probability Scale.

Interaction between warfarin and cranberry juice

Warfarin is extensively used for anticoagulation to a target international normalized ratio of 2.0-3.0 for most indications or 2.5-3.5 for high-risk indications; however, many drugs and dietary supplements induce fluctuations in the international normalized ratio. Such fluctuations may lead to therapeutic failure or bleeding complications. Cranberry juice is increasingly used for the prevention and adjunctive treatment of urinary tract infections. The United Kingdom's Committee on Safety of Medicines has alerted clinicians to a potential interaction between warfarin and cranberry juice and has advised that patients avoid their concurrent use. Review and analysis of the literature revealed that ingestion of large volumes of cranberry juice destabilize warfarin therapy. Small amounts of juice are not expected to cause such an interaction. Clinicians should be aware of this potential interaction and monitor and counsel patients accordingly.