Mephyton (Phytonadione) Drug Interactions: What to Avoid and What to Tell Your Doctor

Phytonadione (the generic form of Mephyton) has a unique interaction profile because it directly affects blood clotting—a system that intersects with many medications. Understanding these interactions is important for your safety, especially if you take warfarin or any other medications.

This guide covers the most clinically important drug interactions with phytonadione. Always tell every member of your healthcare team about all medications, supplements, and vitamins you take.

The Most Important Interaction: Warfarin (Jantoven)

The most critical interaction is between phytonadione and warfarin (Jantoven). Phytonadione directly opposes the anticoagulant effect of warfarin:

Even a small dose of phytonadione can lower your INR and reduce warfarin's effectiveness

Large doses can cause temporary warfarin resistance—meaning when you restart warfarin, higher doses may be needed to reach your therapeutic INR

The resistance is temporary, but your INR must be monitored closely when restarting warfarin after phytonadione

This interaction is intentional when phytonadione is being used therapeutically to reverse warfarin. However, if you are supposed to remain on warfarin and accidentally take too much vitamin K (through supplements or diet changes), it can inappropriately lower your INR and put you at risk for blood clots.

Medications That Reduce Phytonadione Absorption

Because phytonadione is fat-soluble, several medications can reduce how well your body absorbs it from oral tablets:

Bile acid sequestrants (cholestyramine, colestipol): These cholesterol-lowering drugs bind to bile acids in the gut, which are needed for fat-soluble vitamin absorption. Taking them with phytonadione significantly reduces absorption. Take phytonadione at least 2 hours before or 4-6 hours after a bile acid sequestrant.

Orlistat (Alli, Xenical): This weight-loss drug works by blocking fat absorption in the gut. Since phytonadione is fat-soluble, orlistat can significantly reduce how much phytonadione is absorbed. Monitor your INR more closely if taking both.

Mineral oil: Used as a laxative, mineral oil can coat the intestinal wall and reduce absorption of fat-soluble vitamins including vitamin K. Avoid mineral oil if you are on phytonadione therapy.

Medications That Reduce Vitamin K Levels (Affecting Efficacy)

Several medications lower your body's natural vitamin K levels, which can affect how much supplemental phytonadione you may need:

Antibiotics (especially broad-spectrum): Oral antibiotics can alter gut flora that normally produce some vitamin K2. Antibiotics like erythromycin, quinolones, and cephalosporins may affect phytonadione levels or reduce natural vitamin K availability.

Cephalosporins (especially cefamandole, cefotetan): Some cephalosporin antibiotics directly interfere with vitamin K recycling in addition to altering gut flora.

Medications That May Compete for or Affect Anticoagulation Balance

Aspirin and NSAIDs (ibuprofen, naproxen): While these don't directly interact with phytonadione's mechanism, they increase bleeding risk on their own and can complicate the management of patients on warfarin + phytonadione. Discuss all pain relievers with your doctor.

Herbal supplements: Several herbal products can affect anticoagulation balance when taken alongside phytonadione: feverfew, garlic, ginger, ginkgo, and St. John's Wort can all affect clotting or increase anticoagulation. Fennel has also been noted to interact.

Food Interactions: What to Know About Vitamin K in Your Diet

If you take warfarin and are prescribed phytonadione:

Consistent vitamin K intake through diet is critical. Sudden large increases in dietary vitamin K (eating large amounts of spinach, kale, or broccoli) can lower your INR just as phytonadione does

You don't need to avoid vitamin K-rich foods—just keep your intake consistent from week to week

Grapefruit and cranberry juice have been reported to interact with warfarin (though via CYP enzyme effects, not phytonadione's mechanism)—mention these to your prescriber

What to Tell Every Doctor and Pharmacist

Whenever you start or stop phytonadione, make sure all providers who manage your warfarin know—because they'll need to monitor your INR more frequently during the transition. Also tell them about:

All prescription medications, including antibiotics

All over-the-counter medications including aspirin and ibuprofen

All vitamins and herbal supplements, especially any containing vitamin K

Any major changes in your diet, especially leafy green vegetable intake

For information on what side effects to watch for, read our guide on Mephyton side effects.

If you're having trouble filling your phytonadione prescription, medfinder can call pharmacies near you to find where it's available.