A nurse is teaching a client newly prescribed warfarin (Coumadin). Which food interaction must the client understand?

WARFARIN AND VITAMIN K INTERACTION is the most important dietary teaching for anticoagulated patients.

Mechanism: Warfarin works by blocking vitamin K-dependent clotting factors (II, VII, IX, X, Protein C and S); vitamin K in food COMPETES with warfarin's mechanism; more dietary vitamin K = lower anticoagulation effect (lower INR); less dietary vitamin K = higher anticoagulation effect (higher INR and bleeding risk). THE KEY TEACHING POINT — CONSISTENCY, NOT ELIMINATION: Patients do NOT need to avoid vitamin K foods completely; they should maintain a CONSISTENT amount from week to week; the warfarin dose is calibrated to their usual dietary intake.

Examples of high-vitamin k foods: Leafy greens (spinach, kale, Swiss chard); Brussels sprouts; broccoli; cabbage; green tea; parsley; liver.

Scenario: A patient on a stable warfarin dose decides to start a daily green smoothie with 2 cups of spinach — their vitamin K intake spikes → INR drops below therapeutic → thromboembolic event risk. Conversely, they stop eating greens during an illness → INR rises → bleeding risk.

Additional warfarin interactions:

Drugs: Many drugs increase or decrease INR (antibiotics, antifungals, NSAIDs, aspirin, herbal supplements — especially St. John's Wort).

Alcohol: Acute heavy alcohol use elevates INR; chronic alcohol use reduces it.

Illness and diet changes: Any significant change in diet, illness, or new medication requires INR monitoring.

Patient education: Report any new medication (including OTC and herbal) to their provider; monitor for bleeding signs; know their target INR and what to do if they miss a dose.