Grapefruit and Warfarin

Publication Date: 
Tue, 11/08/2011

Grapefruit juice is a popular breakfast option for many patients, especially those interested in improving their overall health. All 12 varieties of grapefruit are rich in vitamin C, which has many documented health benefits. However, grapefruit can affect the action of some medications, including warfarin.1 Researchers at the University of North Carolina concluded that the furanocoumarins, a naturally-occurring substance found in grapefruit juice, had been found to affect the absorption of some medications.

Discuss the consumption of grapefruit and grapefruit products, including juice, with your doctor. A single case study showed that 50 ounces daily was sufficient to increase the patient's INR test results, while a study of a small cohort of patients showed that 24 ounces per day for one week had no significant effect on INR values.1 Individual patient response to grapefruit and grapefruit products may vary.

Food and beverage interactions can be identified sooner by more frequent INR testing. Testing at home allows for both an increased frequency of testing and improved coincidence of the timing of INR testing with the physician's testing instructions. 

  1.  Jellin J.M., et al. Pharmacist's Letter/Prescriber's Letter of Natural Medicines Comprehensive Database. 7th ed. Stockton, CA: Therapeutic Research Faculty. 2005. 626-629.