Vitamin K is a group of fat-soluble vitamins essential for blood clotting and bone health. The name 'vitamin K' derives from the German word 'Koagulation', reflecting its critical role in coagulation processes discovered in the 1930s. This nomenclature persists in medical and scientific literature worldwide, including throughout the UK. Vitamin K exists in several forms—primarily K1 (phylloquinone) and K2 (menaquinones)—each with distinct sources and biological activities. Understanding vitamin K's name, forms, and clinical applications helps patients and healthcare professionals communicate effectively about this essential nutrient and its therapeutic uses.

What Is Vitamin K? Understanding the Name and Types

Vitamin K is a group of fat-soluble vitamins essential for blood clotting and bone health. The 'K' derives from the German word 'Koagulation', reflecting its critical role in coagulation processes discovered by Danish scientist Henrik Dam in the 1930s. This nomenclature has persisted in medical and scientific literature worldwide, including throughout the UK and Europe.

Vitamin K exists naturally in several forms, primarily vitamin K1 (phylloquinone) and vitamin K2 (menaquinones). A synthetic form, vitamin K3 (menadione), also exists but is rarely used in human medicine due to potential toxicity concerns. Each form has distinct sources, absorption characteristics, and biological activities within the body.

The body requires vitamin K to produce specific proteins necessary for blood clotting, including prothrombin (factor II) and factors VII, IX, and X. Without adequate vitamin K, these clotting factors cannot function properly, leading to increased bleeding risk. Beyond haemostasis, there is emerging evidence suggesting vitamin K may play roles in bone mineralisation and cardiovascular health, though research is ongoing to clarify these relationships.

In clinical practice, vitamin K deficiency can occur in newborns (vitamin K deficiency bleeding or VKDB), individuals with malabsorption disorders (such as coeliac disease or inflammatory bowel disease), those taking vitamin K antagonists (such as warfarin), or people with inadequate dietary intake. The NHS recommends that all newborns receive prophylactic vitamin K to prevent potentially serious bleeding complications.

If you experience persistent or unexplained bruising, bleeding gums, nosebleeds, black or tarry stools, or any significant bleeding, seek urgent medical advice via NHS 111 or emergency care as appropriate.

Different Forms of Vitamin K: K1, K2, and K3 Explained

Vitamin K1 (phylloquinone or phytomenadione) represents the predominant dietary form, found abundantly in green leafy vegetables such as kale, spinach, broccoli, and Brussels sprouts. This form accounts for the majority of total vitamin K intake in typical Western diets. Phylloquinone is absorbed in the small intestine alongside dietary fats and transported to the liver, where it primarily supports the synthesis of clotting factors. The bioavailability of K1 from vegetables is relatively modest due to its tight binding within chloroplast membranes, though absorption improves when consumed with dietary fat.

Vitamin K2 (menaquinones) comprises a family of compounds with varying side-chain lengths, designated MK-4 through MK-13. These forms are produced by bacterial fermentation and found in fermented foods (such as natto, a traditional Japanese soy product), certain cheeses, and animal products including egg yolks and meat. Gut bacteria also synthesise menaquinones, though the contribution to overall vitamin K status remains uncertain. Some forms of vitamin K2, particularly MK-7, have been shown to have a longer half-life in circulation compared to K1. Some observational evidence suggests certain K2 forms may support bone and cardiovascular health, though current UK clinical guidelines do not differentiate between forms for specific recommendations.

Vitamin K3 (menadione) is a synthetic, water-soluble precursor that the body can convert to K2. However, menadione is not approved for human use in the UK or EU due to concerns about potential toxicity, including haemolytic anaemia and liver damage at high doses. It may occasionally be used in animal feed but has been superseded by safer vitamin K forms in human therapeutics. When vitamin K supplementation or treatment is required clinically, phytomenadione (K1) remains the standard preparation used in UK medical practice.

Common Brand Names and Supplements Containing Vitamin K

In the UK, pharmaceutical-grade vitamin K is primarily available as phytomenadione under several brand names and formulations. Konakion® MM and Konakion® MM Paediatric are the most commonly used preparations, particularly for newborn prophylaxis and treatment of vitamin K deficiency bleeding. These formulations contain phytomenadione in a mixed micelle vehicle, improving absorption and reducing adverse reactions compared to older preparations. Konakion MM Paediatric is specifically licensed for oral and intramuscular administration in neonates.

For adults requiring vitamin K supplementation or reversal of over-anticoagulation with warfarin, Konakion® MM ampoules (10 mg/mL) are available for intravenous or oral administration. The British Society for Haematology (BSH), local protocols, and the product's Summary of Product Characteristics (SmPC) provide guidance on dosing regimens depending on the clinical indication, INR level, and presence of bleeding. Intravenous administration should be slow, and intramuscular injection is generally avoided in anticoagulated adults. Healthcare professionals must follow these protocols carefully, as inappropriate vitamin K administration can make subsequent warfarin management challenging.

Importantly, vitamin K does not reverse the effects of direct oral anticoagulants (DOACs) such as apixaban or rivaroxaban.

Over-the-counter vitamin K supplements are available in health food shops and pharmacies, typically containing vitamin K1 (phylloquinone) or K2 (often as menaquinone-7, MK-7) in doses ranging from 50–200 micrograms. Many multivitamin preparations also contain vitamin K, usually at levels of 50–75 micrograms per dose.

Important safety consideration: Patients taking warfarin or other vitamin K antagonists should not take vitamin K supplements without medical supervision, as this can significantly interfere with anticoagulation control. Consistent dietary vitamin K intake is preferable to erratic supplementation. Anyone considering vitamin K supplements should discuss this with their GP or pharmacist, particularly if they have clotting disorders, liver disease, or take medications affecting coagulation. Vitamin K supplements are generally well-tolerated, with adverse effects being rare at recommended doses.

If you suspect an adverse reaction to any vitamin K preparation, report it to the MHRA Yellow Card Scheme.

Medical Names: Phytomenadione and Menaquinone

In clinical and pharmaceutical contexts, healthcare professionals use specific chemical nomenclature to describe vitamin K compounds precisely. Phytomenadione (also spelled phytonadione in American English) is the medical name for vitamin K1, the form used therapeutically in UK hospitals and primary care. This term appears on prescription charts, in the British National Formulary (BNF), and in clinical guidance. Phytomenadione and phylloquinone are synonyms that both refer to vitamin K1, though phytomenadione is typically used for medicinal products while phylloquinone often refers to the dietary form.

The mechanism of action of phytomenadione involves serving as a cofactor for gamma-glutamyl carboxylase, the enzyme responsible for post-translational modification of clotting factors. This carboxylation process is essential for these proteins to bind calcium and function properly in the coagulation cascade. When vitamin K is deficient or antagonised (as with warfarin therapy), undercarboxylated clotting factors are produced, resulting in impaired haemostasis.

Menaquinone is the collective medical term for the vitamin K2 family of compounds. Specific menaquinones are designated by their side-chain length: menaquinone-4 (MK-4), menaquinone-7 (MK-7), and so forth. While menaquinones are not routinely used in UK clinical practice for therapeutic purposes, research interest continues regarding their potential roles in bone density and cardiovascular health. Currently, there are no specific recommendations from UK clinical guidelines for menaquinone supplementation for these indications.

When reviewing medical records, prescriptions, or supplement labels, patients may encounter these technical names. Phytomenadione and phylloquinone refer to the same compound (vitamin K1), whilst menaquinone indicates vitamin K2 forms. Understanding this terminology helps patients communicate effectively with healthcare professionals and make informed decisions about supplementation. If uncertainty exists about any vitamin K preparation, patients should consult their GP or pharmacist for clarification, particularly if they have relevant medical conditions or take anticoagulant medications.

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