Does vitamin K1 have side effects?

December 25, 2025 •

3 min read

While most people do not experience adverse effects from oral vitamin K1 at recommended doses, certain circumstances, particularly injections and interactions with specific medications, carry significant risks. This prompts the important question: Does vitamin K1 have side effects?

Quick Summary

Oral vitamin K1 is well-tolerated, but injected forms can cause allergic reactions. Its primary side-effect concern relates to interactions with anticoagulant drugs like warfarin, which require consistent dietary intake.

Key Points

Oral Vitamin K1 is generally safe: Most people experience no side effects from dietary or supplemental oral Vitamin K1 at recommended doses.
Injections carry more risk: Intravenous or intramuscular Vitamin K1 can cause more serious reactions, including anaphylaxis and injection site issues.
Warfarin interaction is critical: Vitamin K1 significantly counteracts the effects of blood thinners like warfarin, requiring consistent dietary intake.
Toxicity is extremely rare: Unlike the synthetic K3, there is no established upper intake level for natural K1, and toxicity is uncommon.
Newborns need careful dosing: High doses of injected Vitamin K in newborns could potentially cause toxicity issues like hemolytic anemia.
Liver and kidney disease precautions: Individuals with impaired liver or kidney function should use caution with Vitamin K supplements.

Understanding Vitamin K1: Phylloquinone

Vitamin K1, also known as phylloquinone, is a fat-soluble vitamin found primarily in plant foods like leafy greens. It is essential for producing proteins required for blood clotting. Vitamin K also includes K2 (menaquinone), made by gut bacteria or found in fermented foods, and the toxic synthetic form K3 (menadione), which is not used in supplements.

Side Effects of Oral Vitamin K1 Supplements

Oral vitamin K1 from diet or supplements at recommended levels is generally safe for most healthy individuals and well-tolerated. The body regulates natural vitamin K forms efficiently, and toxicity from high oral doses is not known, leading regulatory agencies not to set a tolerable upper intake level. Side effects from oral supplements are very rare and typically mild, such as minor digestive upset.

Significant Risks of Injected Vitamin K1

Injected vitamin K1 (phytonadione), used in medical settings, poses a higher risk of side effects compared to oral forms. Reactions at the injection site can include pain, swelling, and redness. More serious, potentially life-threatening anaphylactoid reactions have been reported, especially with intravenous administration. Symptoms of severe allergic reactions can involve itching, hives, rapid heart rate, breathing difficulty, and facial swelling. In newborns, the crucial standard vitamin K shot is safe for preventing bleeding disorders, but excessive doses could potentially cause jaundice or hemolytic anemia.

Critical Interactions with Medications

A major safety concern with vitamin K1 is its interaction with prescription medications, especially blood thinners (anticoagulants). Vitamin K is vital for clotting, so it directly opposes the effects of warfarin. Fluctuations in vitamin K intake can dangerously alter warfarin's effectiveness, necessitating a consistent daily intake and consultation with a doctor before making dietary or supplement changes. Long-term antibiotic use can reduce gut bacteria producing vitamin K. Other drugs, such as bile acid sequestrants (e.g., cholestyramine) and orlistat, can impair the absorption of fat-soluble vitamins like K1.

Who Should Be Cautious with Vitamin K1?

Certain individuals should be cautious with vitamin K supplementation without medical guidance:

Those on warfarin: Consistent vitamin K intake is crucial due to interaction.
Individuals with severe liver disease: Vitamin K may not help clotting issues from severe liver damage, and high doses might worsen the problem.
Kidney patients on dialysis: Injected vitamin K can contain aluminum which may build up in those with impaired kidney function.
Newborns: Require careful dosing of injections to avoid potential toxicity.

High Vitamin K1 Food Sources

Foods particularly rich in vitamin K1 include kale, spinach, broccoli, collard greens, Brussels sprouts, and turnip greens. Consuming these regularly is important for overall health and vital for managing medication interactions.

Comparison of Vitamin K1 Delivery Methods


Feature	Oral Vitamin K1 (Dietary/Supplement)	Injected Vitamin K1 (Phytonadione)
Side Effect Profile	Generally safe with rare, mild digestive issues.	Higher risk of site reactions and severe allergic reactions.
Risk Level	Low, especially at recommended doses.	Higher, particularly with IV administration.
Primary Use	Daily nutritional intake, supplement for mild deficiencies.	Medical treatment for severe vitamin K deficiency or to reverse anticoagulant effects.
Drug Interaction Concern	Significant interaction risk with warfarin, less with other drugs.	Significant interaction risk with warfarin; generally used in controlled medical settings.

Conclusion

The question "Does vitamin K1 have side effects?" has a nuanced answer depending on administration method and individual health. Oral intake from diet or standard supplements is generally very safe. The main risks are associated with injected forms, used for specific medical needs, and critical interactions with anticoagulants like warfarin. Individuals with liver or kidney disease, or those taking blood thinners, must consult their doctor before altering vitamin K intake to ensure safety.

For More Information

For a detailed overview of vitamin K and its health implications, please visit the NIH Office of Dietary Supplements website.

Frequently Asked Questions

For most healthy individuals, oral vitamin K1 from food or standard supplements is very safe. However, people with certain medical conditions, such as liver or kidney disease, or those on specific medications like warfarin, need to exercise caution and consult a healthcare provider.

Oral vitamin K1 rarely causes side effects, which are typically mild digestive issues. Injected vitamin K1 carries a higher risk of more severe reactions, including pain at the injection site and potentially life-threatening allergic responses like anaphylaxis.

Yes, vitamin K1 can have a serious interaction with warfarin, as it directly counteracts the anticoagulant effect. Patients taking warfarin must maintain a consistent daily intake of vitamin K to prevent dangerous changes in blood clotting.

Toxicity from natural vitamin K1 is extremely rare, so there is no established Tolerable Upper Intake Level (UL) for it. However, excessive doses in injected form, particularly in newborns, can cause harm.

The standard vitamin K injection for newborns is considered very safe and helps prevent a serious bleeding disorder. While some temporary pain or swelling at the injection site may occur, serious side effects are extremely rare at the recommended dosage.

Besides the major interaction with warfarin, vitamin K1 can also interact with certain antibiotics, bile acid sequestrants used for cholesterol (like cholestyramine), and the weight-loss drug orlistat.

Neither natural vitamin K1 nor K2 is known to have toxicity at high doses. The toxicity concern is specifically related to the synthetic form, vitamin K3 (menadione), which is not used in supplements.