Vitamin K1 (Phylloquinone)
Also known as: Phylloquinone, Vitamin K, Phytonadione
The clotting vitamin from leafy greens — essential for blood coagulation, with a famous warfarin interaction.
In short
Vitamin K1 (phylloquinone) is the dietary form of vitamin K — abundant in leafy greens — and its essential job is blood clotting: it activates the liver proteins that let blood coagulate, which is why every newborn in the US receives a vitamin K injection at birth to prevent bleeding. Deficiency in adults is rare outside specific conditions, but one interaction defines vitamin K's medical reputation: it directly antagonizes warfarin, the classic blood thinner, so people on warfarin must keep their K intake stable. Its bone-health credentials are real but mostly belong to the K2 form.
- Activates clotting factors — the 'koagulation' vitamin
- Rich in leafy greens: kale, spinach, collards
- Newborns receive a K injection at birth to prevent bleeding
- Directly antagonizes warfarin — keep intake stable on warfarin
What is Vitamin K1 (Phylloquinone)?
Vitamin K's name comes from its discovery story — 'Koagulationsvitamin' — and coagulation remains its defining job. K1, the plant form, activates clotting factors in the liver through a precise carboxylation cycle, making it one of the few vitamins with a directly observable function: without it, blood does not clot properly. That singular role explains both its clinical uses — the universal newborn injection, warfarin reversal — and its one famous danger: for people on warfarin, a sudden kale-salad habit is a genuine drug interaction, not a wellness cliché.
Where it comes from
Natural sources
- Phylloquinone in green leafy vegetables: kale, spinach, collards, broccoli
- Also in some plant oils (soybean, canola)
- Gut bacteria produce the K2 form (menaquinones) separately
How it's made
Supplements use phytonadione (synthetic K1) for deficiency and warfarin-related uses.
What it does in products
Benefits
Normal blood clotting
strong evidenceVitamin K1 is essential for coagulation; deficiency causes bleeding. The newborn K injection has made vitamin K deficiency bleeding a rarity.
Warfarin reversal and anticoagulation management
strong evidenceK1 (phytonadione) is the medical antidote to warfarin overdose and the key variable in stable warfarin dosing.
Bone health
moderate evidenceVitamin K status correlates with bone outcomes, and K activates osteocalcin — but the supplementation evidence is stronger for the K2 form (MK-7).
Side effects & safety
Safety considerations
- Safe and necessary — the newborn injection is standard care; oral supplements in pregnancy are rarely needed.
- Antagonizes warfarin (Coumadin) — keep dietary intake consistent and discuss any supplement with your prescriber.
- Some antibiotics disrupt gut K production and K status.
- Not for: Unstable warfarin management without medical coordination
How it's used
In supplements
Adult adequate intake: 120 mcg/day (men), 90 mcg/day (women). Supplements are usually unnecessary given dietary abundance; medical dosing (deficiency, warfarin reversal) is clinician-directed.
Regulatory status
- Dietary supplement under DSHEA; injectable phytonadione is a prescription drug (newborn prophylaxis, warfarin reversal).
- GRAS (generally recognized as safe) for food use.
Popular products containing Vitamin K1 (Phylloquinone)
- Super K (K1 + K2)Life Extension · Supplement
- Vitamin K (K1)Solgar · Supplement
- Vitamin K ComplexNOW Foods · Supplement
Product names and brands are shown for reference and belong to their respective owners.
K1 vs K2: the vitamin K division of labor
Vitamin K is one name for two molecules with different jobs. K1 (phylloquinone) goes to the liver and runs the clotting system — its natural priority. K2 (menaquinones, especially MK-7) circulates to bones and blood vessels, activating osteocalcin and matrix Gla protein — the forms behind bone- and arterial-health claims. The division explains the supplement landscape: K1 products are for clotting-related medical use, K2 products target bone and cardiovascular support. Both are safe at supplement doses, and combination products (K1 + K2, often with vitamin D) cover both functions.
Frequently asked questions
Why do newborns get a vitamin K shot?
Newborns are born with very low vitamin K and sterile guts that cannot yet synthesize K2, putting them at risk of potentially fatal bleeding. A single intramuscular K1 injection at birth prevents vitamin K deficiency bleeding — one of the safest, most effective interventions in pediatrics.
Can I take vitamin K if I am on warfarin?
Vitamin K directly opposes warfarin, so it is not a casual supplement in that context — but consistency is the real issue. Work with your prescriber: the goal is a stable K intake, not zero K. Never self-dose K to 'fix' an INR.
Which foods are highest in vitamin K1?
Dark leafy greens dominate: kale, collards, spinach, and Swiss chard, followed by broccoli and Brussels sprouts. One serving of kale far exceeds the daily adequate intake.
The bottom line
Vitamin K1 is the essential clotting vitamin — abundant in greens, medically vital at birth, and the key interaction variable for anyone on warfarin. Most people get plenty from food; the supplement market is mostly about K2 for bones and arteries.
References
- Vitamin K fact sheet for health professionals. NIH Office of Dietary Supplements. https://ods.od.nih.gov/factsheets/VitaminK-HealthProfessional/
- Vitamin K and blood clotting — literature search. PubMed (National Library of Medicine). https://pubmed.ncbi.nlm.nih.gov/?term=vitamin+K+coagulation+factors
- Newborn vitamin K prophylaxis — literature search. PubMed (National Library of Medicine). https://pubmed.ncbi.nlm.nih.gov/?term=vitamin+K+newborn+prophylaxis
- Vitamin K1 compound summary. PubChem (National Library of Medicine). https://pubchem.ncbi.nlm.nih.gov/compound/5284607