Vitamin ·Strong evidence ·Reviewed May 2026
The primary form of vitamin K found in plants, essential for blood clotting. Also supports bone health to a lesser extent than K2.
The bottom line
Evidence rating strong. Most-documented uses: blood clotting, bone health, wound healing. 22 sources indexed (1999–2025), with 9 interaction records on file.
Core mechanism
Serves as a cofactor for gamma-glutamyl carboxylase, which activates clotting factors II (prothrombin), VII, IX, and X in the coagulation cascade.
Fat-soluble; abundant in green leafy vegetables1,2
Dosing protocol
Patients on warfarin must keep intake stable, not necessarily low. Fat-soluble; pair with fish oil or meal fat.
Ranked by evidence and value.
Real-world pricing across three quality tiers. Assumes Phylloquinone.
Assumes 90-120 mcg/day. Vendor basis: NOW/iHerb, Vitacost, Life Extension, and Amazon marketplace; standalone K1 remains inexpensive. Updated 2026-05-28.
How much you'd eat to match a supplemental dose.
Leafy greens are the dominant food source of K1.
Food can easily cover this unless greens intake is extremely low.
What to test, the optimal window inside the conventional range, and how long a response takes.
Adequate K1 intake keeps INR in normal range; deficiency or warfarin therapy pushes INR upward.5,18
Direct serum K1 assays exist (research labs) but PT/INR is the functional clinical marker. Patients on warfarin require careful K1 dose stability.
Where this appears in the symptom-to-supplement map, ranked by relevance.
Vitamin K1 is essential for clotting factor synthesis, so correcting a true deficiency can reduce abnormal bruising.1,2
Do not start if you are on warfarin without clinician guidance, since K1 directly opposes it.
Vitamin K supports clotting and capillary integrity, and topical use is explored for circles driven by blood pooling and vessel leak.1,2
Evidence is mostly topical and from small studies; oral K1 has little direct data for this.
Vitamin K1 supports clotting factor activity, modestly relevant in heavy menstrual bleeding.1,2
Not a substitute for evaluation of fibroids, adenomyosis, or hormonal causes.
Vitamin K1 supports normal clotting factor production, which is relevant only when bleeding stems from a vitamin K deficiency.1,2
Do not start if you take warfarin, since vitamin K1 interferes with it; confirm any clotting concern with a clinician.
Vitamin K intake has special relevance for people using warfarin because VKORC1 and CYP2C9 genotype influence warfarin sensitivity and dose requirements (PMID 20653669).
Recommendation: Keep vitamin K intake consistent and coordinate any vitamin K supplement with the prescriber if taking warfarin or another vitamin K antagonist.
K1 primarily supports coagulation while K2 (MK-7) directs calcium to bones. Complementary roles in vitamin K metabolism.
Recommendation: Both forms are beneficial. K2 (MK-7) has a longer half-life and better extrahepatic distribution than K1.
High-dose vitamin E (>400 IU) can antagonize vitamin K-dependent clotting factor activation, increasing bleeding risk.
Recommendation: Keep vitamin E under 400 IU if taking K1 for coagulation support or if on anticoagulant therapy. Monitor INR.
K1 primarily supports coagulation while K2 (MK-7) directs calcium to bones. Complementary roles in vitamin K metabolism.
Recommendation: Both forms are beneficial. K2 (MK-7) has a longer half-life and better extrahepatic distribution than K1.
MCT Oil provides a fat-containing carrier that can improve absorption of fat-soluble compounds like Vitamin K1.
Recommendation: Take Vitamin K1 with MCT Oil or another fat-containing meal to improve absorption.
Flaxseed Oil provides a fat-containing carrier that can improve absorption of fat-soluble compounds like Vitamin K1.
Recommendation: Take Vitamin K1 with Flaxseed Oil or another fat-containing meal to improve absorption.
Evening Primrose Oil provides a fat-containing carrier that can improve absorption of fat-soluble compounds like Vitamin K1.
Recommendation: Take Vitamin K1 with Evening Primrose Oil or another fat-containing meal to improve absorption.
Ginkgo's antiplatelet activity can mildly counter vitamin K1's pro-coagulant role; relevant for warfarin patients managing INR through K1 intake.
Recommendation: For warfarin patients, keep both vitamin K1 intake and ginkgo dose stable. Monitor INR closely if either changes.
Stacking a vitamin K1 supplement on top of chlorella meaningfully increases total vitamin K1 exposure. For most healthy people this is simply additive and unremarkable, but the combined load becomes clinically relevant for anyone whose vitamin K status is being deliberately managed (for example, people on warfarin or other vitamin K antagonists, or those undergoing INR monitoring), where the two sources together can blunt anticoagulation and destabilize INR more than either alone.
Recommendation: For healthy users with no clotting concerns, no special action is needed beyond awareness that chlorella already supplies substantial vitamin K1, so an additional K1 supplement may be redundant. Anyone on warfarin or another vitamin K antagonist should keep total vitamin K intake (chlorella plus any K1 supplement) consistent day to day rather than starting, stopping, or fluctuating doses, and should have INR checked after any change. Keep the daily chlorella dose stable (commonly 2 to 5 g) and discuss the combination with the prescribing clinician before adding supplemental K1.
Vitamin K1 (phytonadione) directly reverses warfarin's anticoagulant effect. Even small, regular doses found in multivitamins (25 mcg/day) have been shown to lower the INR and require warfarin dose increases, particularly in patients with depleted baseline vitamin K stores. Inconsistent K1 intake from supplements is one of the most common causes of unstable INR.
Recommendation: Avoid vitamin K1 supplements unless your prescriber has specifically directed you to take them. If a multivitamin containing vitamin K1 is unavoidable, keep the dose and brand identical every day and ask your anticoagulation clinic to re-check INR within 1-2 weeks of starting.
Numbered references. Citations throughout the page link here.
Vitamin K decreases general fracture risk (OR 0.42 vertebral, OR 0.44 clinical); can be an option to counter bone loss disorders
Four cohort studies and one nested case-control study (1,114 fracture cases, 80,982 participants) examined vitamin K1 intake and fracture risk
Pooling 7 trials: OR 0.40 for vertebral fractures, OR 0.23 for hip fractures, OR 0.19 for all nonvertebral fractures favoring vitamin K
Dezee KJ, Shimeall WT, Douglas KM et al.. Treatment of excessive anticoagulation with phytonadione (vitamin K): a meta-analysis. Archives of internal medicine. 2006
Macias-Cervantes HE, Ocampo-Apolonio MA, Guardado-Mendoza R et al.. Effect of vitamin K1 supplementation on coronary calcifications in hemodialysis patients: a randomized controlled trial. Journal of nephrology. 2025
Saritas T, Reinartz S, Krüger T et al.. Vitamin K1 and progression of cardiovascular calcifications in hemodialysis patients: the VitaVasK randomized controlled trial. Clinical kidney journal. 2022
Lubetsky A, Yonath H, Olchovsky D et al.. Comparison of oral vs intravenous phytonadione (vitamin K1) in patients with excessive anticoagulation: a prospective randomized controlled study. Archives of internal medicine. 2003
Patel RJ, Witt DM, Saseen JJ et al.. Randomized, placebo-controlled trial of oral phytonadione for excessive anticoagulation. Pharmacotherapy. 2000
Smith JM, Smith CK, Zhu X et al.. Supplementation of Vitamin K1 in Dogs With Chronic Enteropathy. Journal of veterinary internal medicine. 2025
EFSA Panel on Additives and Products or Substances used in Animal Feed (FEEDAP), Bampidis V, Azimonti G et al.. Safety and efficacy of an additive consisting of synthetic vitamin K(1) (phytomenadione) for horses (JARAZ Enterprises GmbH & Co. KG). EFSA journal. European Food Safety Authority. 2021
Vitamin K required for osteocalcin carboxylation regulating bone mineral accretion; promotes osteoblast transition to osteocytes and limits osteoclastogenesis
Dietary vitamin K can decrease therapeutic effectiveness of warfarin; poor vitamin K status increases sensitivity to small changes in vitamin K intake
Shields RC, McBane RD, Kuiper JD et al.. Efficacy and safety of intravenous phytonadione (vitamin K1) in patients on long-term oral anticoagulant therapy. Mayo Clinic proceedings. 2001
von Kries R. Oral versus intramuscular phytomenadione: safety and efficacy compared. Drug safety. 1999
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