Mephyton (Phytonadione) Side Effects: What to Expect and When to Call Your Doctor

One of the most reassuring things about oral phytonadione (the generic form of Mephyton) is its side effect profile: the oral tablet form is generally very well tolerated, and side effects are rare. However, there are important safety considerations—especially related to the injectable form and to the interaction with warfarin.

This guide covers what to expect when taking phytonadione and when to contact your healthcare provider.

Common Side Effects of Oral Phytonadione (Mephyton)

Oral phytonadione tablets have very few documented side effects. The most commonly reported include:

Taste changes (rare, occasionally reported with phytonadione)

Mild stomach upset or nausea (uncommon)

Skin rash or itching (rare hypersensitivity reaction)

For the vast majority of patients taking the oral tablet, no significant side effects occur. This makes phytonadione one of the more tolerable prescription medications.

Serious Side Effects and When to Seek Immediate Help

The most important serious side effects of phytonadione are related to:

Allergic reaction: Seek emergency help immediately if you experience skin rash, hives, swelling of the face, lips, tongue or throat, or difficulty breathing. These are signs of a severe allergic reaction.

Bluish discoloration: Bluish color of the fingernails, lips, skin, palms, or nail beds is a warning sign that should be evaluated immediately.

Blood clotting problems: If you are taking phytonadione to reverse warfarin, taking too high a dose may restore conditions that originally caused your blood clotting risk. Signs of a clot include sudden pain, swelling, redness, warmth in the leg; sudden shortness of breath; or chest pain. Contact your doctor immediately.

Injectable Phytonadione: Much Higher Risk Profile

While oral phytonadione is well tolerated, the injectable form has a significantly more serious safety profile. Severe hypersensitivity reactions—including anaphylaxis and death—have been reported with IV phytonadione administration. Most of these occurred with intravenous (IV) administration.

Side effects reported with injectable phytonadione include:

Anaphylactoid reactions (sudden severe allergic-type response)

Flushing sensations

Peculiar taste sensation

Dizziness

Rapid, weak pulse

Profuse sweating

Brief hypotension (sudden drop in blood pressure)

Dyspnea (shortness of breath)

Cyanosis (bluish discoloration)

This is why IV phytonadione should only be administered in a monitored clinical setting, and only when the clinical benefit clearly outweighs the risk.

Neonatal Considerations

In newborns, phytonadione has been associated with hemolysis, jaundice, and hyperbilirubinemia, particularly in premature infants and primarily at doses above those recommended. These are monitored carefully in the hospital setting where neonatal phytonadione is given.

The Warfarin Resistance Risk

One important clinical consideration: if you take phytonadione to reverse warfarin, especially in large doses, you may develop temporary resistance to warfarin. This means when your doctor restarts warfarin (if you need it ongoing), higher doses may initially be required to achieve the target INR. This effect is temporary but must be monitored carefully.

When to Call Your Doctor

Call your doctor if you experience:

Signs of an allergic reaction (rash, swelling, difficulty breathing)

Signs of a blood clot (leg pain/swelling, shortness of breath, chest pain)

Your INR is not responding to treatment after 12-48 hours as expected

You notice new or worsening bleeding symptoms

To understand what other drugs interact with phytonadione and might affect your safety, read our guide on Mephyton drug interactions.

If you're having trouble filling your phytonadione prescription, medfinder can help locate a pharmacy near you that has it in stock.