Black licorice ingestion: Yet another confounding agent in patients with melena.

We describe an 80-year-old woman with atrial fibrillation, anti-coagulated with warfarin, who on two separate occasions developed black tarry stools and an elevated international normalized ratio (INR) after eating a pound of Black Licorice. During her most recent episode, her hematocrit was 14 (baseline 34) and her INR was 5.5 (baseline 2.1). She was advised to restrict licorice consumption, and a follow-up INR two weeks later was 1.2. Black Licorice is derived from the root of the plant, Glycyrrhiza glabra. The components of its extract inhibit the P450 system enzymes that metabolize Warfarin, inhibit thrombin, and prolong fibrinogen clotting times. Hence, the anti-thrombotic activity and inhibition of warfarin metabolism might synergistically amplify anti-coagulation. The presence of Black Licorice in the stool can also mimic melena and confound its clinical presentation. Health care providers should caution patients who are at risk for bleeding or on warfarin to avoid black licorice due to an elevated risk of gastrointestinal bleeding.