Vitamin K2 in Ireland: Benefits, D3 + K2, MK-4 vs MK-7, Dosage and Safety

Vitamin K2 is now sold on its own and paired with vitamin D3 in most Irish pharmacies and health stores, usually with a confident story about bones and arteries. This guide separates what randomised trials have shown from what remains theory, explains the difference between MK-4 and MK-7, and sets out how to choose a vitamin K2 or D3 + K2 supplement in Ireland.

Vitamin K2 (menaquinone) definition: Vitamin K2 is the family of menaquinones (MK-4 to MK-13), a fat-soluble form of vitamin K produced by bacteria and found in fermented foods such as natto and cheese and, as MK-4, in animal tissues, where it acts as a cofactor for the enzyme that activates vitamin K-dependent proteins in liver, bone and blood vessel walls.

What is vitamin K2? K1 vs K2, MK-4 vs MK-7

Vitamin K is a group of fat-soluble compounds that the body needs to activate a set of proteins through a chemical step called carboxylation. The HSE describes vitamin K as needed for blood clotting and wound healing, and notes some evidence that it may support bone health.^[14]

Vitamin K1 (phylloquinone) is the plant form, found in green leafy vegetables, and makes up most of the vitamin K in an Irish diet. Vitamin K2 (menaquinones) is made by bacteria. The two supplement forms are MK-4, a short-chain menaquinone found in animal foods and usually made synthetically, and MK-7, a long-chain menaquinone fermented from natto.

The practical difference is how long each form stays in the blood. Schurgers and colleagues at Maastricht University (Blood, 2007, PMID 17158229) compared K1 and MK-7 in healthy volunteers and found MK-7 had a half-life of around three days against hours for K1, reached far higher steady-state levels and activated the bone protein osteocalcin more effectively at the same dose.^[5]

MK-4 behaves differently again. Sato et al. (Nutrition Journal, 2012, PMID 23140417) gave healthy women a single 420 µg dose of either MK-4 or MK-7. MK-7 was well absorbed and raised serum levels; MK-4 was not detectable in serum at all.^[6] This matters when reading a D3 + K2 label: 45 µg of MK-4 and 100 µg of MK-7 are not comparable doses.

Why vitamin D3 and K2 are taken together

The logic sold with the D3 + K2 pairing is simple: vitamin D increases how much calcium you absorb, and vitamin K activates the proteins that decide where that calcium goes. Each half of that sentence is backed by an authorised EU health claim. Vitamin D contributes to normal absorption and utilisation of calcium and phosphorus and to the maintenance of normal bones; vitamin K contributes to normal blood clotting and to the maintenance of normal bones.^[15]

The claim that the two are better together is a different thing, and it has not been established in a human outcome trial. A large randomised trial of a D3 + K2 combination, AVADEC (Diederichsen et al., Circulation, 2022, PMID 35465686), gave 365 Danish men 720 µg MK-7 plus 25 µg vitamin D or placebo for two years and found no difference in calcification progression. Because AVADEC had no vitamin D-only arm, it cannot say whether adding K2 to D3 helps or not; it only says the combination did not beat placebo on calcification. The Rønn bone trial added MK-7 on top of vitamin D and calcium in both arms and found no extra bone density benefit over three years.^[3] The positive Knapen bone trial used MK-7 alone, not a combination.

So the honest position is: taking K2 alongside D3 is reasonable and well tolerated, vitamin D3 has its own strong case in Ireland, and K2 has its own moderate case for bone in postmenopausal women. Treat the combination as two supplements that happen to share a capsule, not as a proven synergy. Our vitamin D3 Ireland guide covers the vitamin D side in depth, including deficiency rates and HSE dosing.

Vitamin K2 benefits: what the trials show

Most "vitamin K2 benefits" pages list bones, arteries, teeth and more without saying which were tested in people. Here is the evidence that actually exists, including the trials that found nothing.

Bone

Arteries and the heart

Vitamin K2 foods

MK-7 intake from food is likely to be limited in diets that contain little or no natto, its richest dietary source, and natto, a Japanese fermented soybean dish, is almost absent from Irish shops and kitchens. The HSE lists vitamin K sources as green leafy vegetables, vegetable oils and cereal grains, with small amounts in meat and dairy; the vegetable sources are K1.^[14]

In Dutch cohorts, cheese was the main dietary source of menaquinones, which is why the Rotterdam and Beulens studies could compare high and low intakes at all.^[8] Egg yolk, butter and liver contribute MK-4. Kefir and sauerkraut are often listed online as K2 sources, but published values are low and variable and should not be relied on for intake.

How much vitamin K2 to take with D3

There is no official Irish or EU recommended intake specific to vitamin K2. EFSA set an adequate intake of 70 µg/day for total vitamin K in adults (EFSA Journal, 2017) and declined to set an upper intake level because no adverse effects were identified.^[12] The HSE puts the figure at about 1 µg per kilogram of body weight per day and says 1 mg or less from supplements is unlikely to cause harm.^[14]

The trial doses are the useful reference. Theuwissen et al. (British Journal of Nutrition, 2012, PMID 22289649) tested MK-7 at 10, 20, 45, 90, 180 and 360 µg/day for 12 weeks in 42 healthy adults. Doses of 90 µg and above improved carboxylation of osteocalcin and matrix Gla protein; 45 µg and below did not reach significance. No dose affected thrombin generation.^[7] The three-year bone and arterial trials used 180 µg/day.^[1]

For vitamin D, the HSE position is the clearest guidance in Ireland: everyone needs a supplement. Adults aged 13 to 64 should take 15 µg (600 IU) daily from 31 October to 17 March, or all year if they have darker skin, limited sun exposure or are pregnant; adults aged 65 and over should take 15 µg daily all year round.^[13] EFSA's tolerable upper intake level for adults is 100 µg (4,000 IU) per day.^[11]

There is no evidence-based ratio of K2 to D3. The 1,000 IU D3 to 45 µg MK-4 pairing in many combination capsules reflects formulation convention, not a trial. The MK-7 trials discussed here used 90 to 375 µg/day, with the positive three-year bone trial using 180 µg/day. These are research doses, not an official Irish or EU recommended intake for K2, and they do not establish a personal dose for anyone. Vitamin K is fat-soluble, so trials gave it with food.

Vitamin K2 side effects, warfarin and safety

Vitamin K2 has one of the cleaner safety records among supplements. EFSA found no adverse effects at any studied intake and did not set an upper level for vitamin K.^[12] In AVADEC, two years of 720 µg/day MK-7 plus vitamin D produced the same adverse event rate as placebo, with no excess of severe bleeding, venous thromboembolism or cancer.^[3] Theuwissen et al. found no effect on thrombin generation at up to 360 µg/day.^[7]

A common search is "can vitamin K2 cause blood clots". In people not on anticoagulants, no increased clotting risk was detected in the cited trials: extra MK-7 did not change thrombin generation in Theuwissen 2012, and venous thromboembolism rates were the same as placebo in AVADEC. Those findings apply to the doses and populations studied; they do not rule out every possible risk for every person.

Vitamin K2 in Ireland: HSE advice, FSAI status, VAT

Ireland has a specific reason for the winter D3 + K2 habit. The Irish Longitudinal Study on Ageing (Laird et al., Journal of Gerontology, 2018, PMID 28958047) found one in eight adults over 50 were vitamin D deficient (below 30 nmol/L) overall, rising to one in four in winter and to 37% of those over 80 in winter.^[10] That is why the HSE now advises people aged 13 to 64 to take 15 µg vitamin D daily between Halloween and St Patrick's Day, year-round for specified groups, and adults aged 65 and over to take it all year.^[13]

There is no equivalent Irish deficiency data or public health advice for vitamin K2. The HSE position on vitamin K is simply that most people get enough from food and that supplements at 1 mg/day or less are unlikely to cause harm.^[14] K2 in Ireland is therefore a personal choice based on the postmenopausal bone evidence, not a public health recommendation.

How to choose a vitamin K2 or D3 + K2 supplement in Ireland

Four questions settle most purchases.

1. Which K2 form? If you want the form with long-term trial data, choose MK-7 at 90 to 180 µg. MK-4 at 45 µg is the form in most D3 + K2 combination capsules; it is a legitimate vitamin K source and carries the same authorised claims, but it has no bone or vascular trial data at that dose and is poorly detected in serum.^[6]

2. Separate or combined? A combined capsule is convenient for a winter routine at the HSE vitamin D level. Separate products let you set the D3 dose independently and use an MK-7 dose that matches the trials. If you already take a vitamin D3 product from our vitamin D3 range, adding a standalone MK-7 is the simplest route.

3. Capsule type. Healthy Origins uses a vegetarian (tapioca) softgel with beeswax, NOW uses a hypromellose veg capsule, Jarrow uses bovine gelatin. Vegans should note the beeswax in Healthy Origins and the gelatin in Jarrow.

4. Other supplements you take. Vitamin D works alongside magnesium and calcium in bone, and magnesium is the mineral Irish adults most often ask about next; see which magnesium is best for the forms compared. Omega-3 and vitamin K2 are both fat-soluble and are often taken with the same meal; our omega-3 Ireland guide covers EPA and DHA dosing.

Frequently asked questions

What is vitamin K2 and how is it different from vitamin K1?

Vitamin K2 is the menaquinone family of vitamin K, found in fermented foods such as natto and cheese and in some animal foods, while vitamin K1 (phylloquinone) comes mainly from green leafy vegetables. Both support the same vitamin K-dependent proteins, but Schurgers et al. (Maastricht University, Blood, 2007, PMID 17158229) showed the long-chain K2 form menaquinone-7 (MK-7) stays in the blood for days rather than hours and raises the activation of bone and vascular proteins more efficiently than the same dose of K1. The HSE lists vitamin K as needed for normal blood clotting and notes some evidence for bone health.

Do I need to take vitamin K2 with vitamin D3?

You do not have to. Vitamin D3 and vitamin K2 each have their own authorised EU health claims (vitamin D for normal calcium absorption and normal bones; vitamin K for normal blood clotting and normal bones) and each works whether or not the other is taken. The pairing rationale is mechanistic: vitamin D increases calcium absorption and vitamin K activates osteocalcin, which binds calcium in bone, and matrix Gla protein, which is proposed to limit calcium deposits in soft tissue. A large trial of the combination, AVADEC (Diederichsen et al., Circulation, 2022, PMID 35465686), found that 720 µg MK-7 plus 25 µg vitamin D for two years did not slow calcification progression versus placebo; it had no vitamin D-only arm, so the added benefit of K2 over vitamin D alone remains untested in humans.

What are the benefits of vitamin K2 according to clinical trials?

The strongest human evidence is for bone in postmenopausal women. Knapen et al. (Maastricht University, Osteoporosis International, 2013, PMID 23525894) gave 244 healthy postmenopausal women 180 µg/day of MK-7 or placebo for three years and found MK-7 reduced the age-related decline in bone mineral density at the lumbar spine and femoral neck, but not at the total hip. The same group (Thrombosis and Haemostasis, 2015, PMID 25694037) reported reduced arterial stiffness markers over three years. The evidence is mixed: Rønn et al. (Osteoporosis International, 2021, PMID 33030563) found 375 µg/day MK-7 for three years did not improve bone density in 142 women with osteopenia, and Emaus et al. (Osteoporosis International, 2010, PMID 19937427) found 360 µg/day for one year had no effect in early menopausal women. No MK-7 trial has measured fractures or cardiovascular events.

What is the difference between vitamin K2 MK-4 and MK-7?

MK-4 and MK-7 are both vitamin K2 but behave differently in the body. Sato et al. (Nutrition Journal, 2012, PMID 23140417) found a single 420 µg dose of MK-4 was not detectable in serum, while MK-7 at the same dose was well absorbed and raised serum levels, and Schurgers et al. (Blood, 2007, PMID 17158229) reported a half-life of about three days for MK-7 compared with hours for K1. The MK-7 trials that showed bone effects used 180 µg/day; the MK-4 bone studies used pharmacological doses of 45 mg/day in Japan, about a thousand times higher than the 45 µg found in typical D3 + K2 capsules. Low-dose MK-4 is therefore the least studied supplement form.

How much vitamin K2 should I take with D3?

Human trials of MK-7 used 90 to 375 µg per day, and the positive three-year bone and arterial trials used 180 µg per day; these are research doses, not an official Irish or EU recommended intake for K2. Theuwissen et al. (British Journal of Nutrition, 2012, PMID 22289649) found MK-7 doses of 90 µg and above improved vitamin K status markers within 12 weeks, with no effect on clotting measures in healthy adults. For vitamin D, the HSE advises people aged 13 to 64 in Ireland to take 15 µg (600 IU) daily from 31 October to 17 March, year-round for specified groups, and adults aged 65 and over to take 15 µg daily all year. EFSA's tolerable upper intake level for vitamin D is 100 µg (4,000 IU) per day for adults, a ceiling rather than a target. There is no fixed required ratio of K2 to D3.

What are the side effects of vitamin K2?

Vitamin K2 at supplement doses is well tolerated in trials. The HSE states that taking 1 mg (1,000 µg) or less of vitamin K supplements a day is unlikely to cause harm, and EFSA has not set an upper intake level for vitamin K because no adverse effects have been identified at dietary and supplemental intakes. The AVADEC trial (Circulation, 2022, PMID 35465686) found adverse event rates with 720 µg/day MK-7 plus vitamin D were the same as placebo over two years. The important exception is people taking vitamin K antagonist anticoagulants such as warfarin, where vitamin K2 directly opposes the medicine and must not be started without medical advice.

Can you take vitamin K2 if you are on warfarin or blood thinners?

Not without your prescriber's agreement. Warfarin works by blocking vitamin K recycling, so added vitamin K2 lowers its effect. Schurgers et al. (Blood, 2007, PMID 17158229) showed that MK-7 interfered with anticoagulation at lower doses than vitamin K1 because of its long half-life. Anyone on warfarin or another vitamin K antagonist in Ireland should speak to their GP, anticoagulation clinic or pharmacist before taking any vitamin K supplement, including D3 + K2 combinations. Direct oral anticoagulants such as apixaban and rivaroxaban do not work through vitamin K, but you should still check with your prescriber.

Which foods are high in vitamin K2?

Natto (fermented soybeans) is by far the richest food source of vitamin K2, specifically MK-7. Hard and soft cheeses, egg yolk, butter, liver and other animal foods provide smaller amounts, mostly as MK-4 and longer-chain menaquinones. In the Rotterdam Study (Geleijnse et al., Journal of Nutrition, 2004, PMID 15514282), cheese was the main source of menaquinone in the Dutch diet. Green leafy vegetables supply vitamin K1, not K2. A typical Irish diet is low in natto and so provides little MK-7.

Is vitamin K2 legal in Ireland?

Yes. Vitamin K2 as menaquinone-7 and menaquinone-4 is a permitted vitamin form in food supplements across the EU, and products containing it are sold in Ireland as food supplements under FSAI (Food Safety Authority of Ireland) oversight, not as medicines. Two authorised EU health claims apply to vitamin K under Regulation (EU) 432/2012: it contributes to normal blood clotting and to the maintenance of normal bones. Food supplements in Ireland carry 13.5% VAT, confirmed in the Revenue Tax and Duty Manual on food supplement products.

Does vitamin K2 reverse calcification of the arteries?

No human trial has shown that. The idea comes from observational studies: the Rotterdam Study (Geleijnse et al., 2004, PMID 15514282) and Beulens et al. (Atherosclerosis, 2009, PMID 18722618) found higher dietary menaquinone intake was associated with less coronary calcification and lower coronary heart disease risk, but association is not cause. The largest randomised test, AVADEC (Diederichsen et al., Circulation, 2022, PMID 35465686), gave 365 men with existing aortic valve calcification 720 µg MK-7 plus vitamin D for two years and found no significant difference in valve or coronary calcification progression versus placebo. Vitamin K2 should not be taken on the expectation of reversing existing calcification.

How long does vitamin K2 take to work?

Blood markers of vitamin K status change within weeks: Theuwissen et al. (British Journal of Nutrition, 2012, PMID 22289649) saw improved carboxylation of osteocalcin and matrix Gla protein within 12 weeks at 90 µg MK-7 and above. Measurable bone outcomes take far longer. In the three-year Knapen trial (Osteoporosis International, 2013, PMID 23525894), both groups lost bone in year one and the difference between MK-7 and placebo only became significant after two to three years, but a second three-year trial (Rønn et al., 2021, PMID 33030563) found no bone density difference at any point. One-year trials, including Emaus et al. (2010, PMID 19937427), found no bone effect.

Where can I buy vitamin K2 and D3 + K2 supplements in Ireland?

Probiotic.ie, a Dublin-based Irish retailer, stocks Healthy Origins Vitamin K2 as MK-7 100 µg (180 softgels, from natto), Jarrow Formulas Vitamin K2 MK-7 90 µg (60 softgels, VitaMK7), NOW Vitamin D3 & K2 1,000 IU / 45 µg MK-4 (120 veg capsules) and Life Extension Vitamins D and K with Sea-Iodine, all shipped from Dublin with tracked nationwide delivery across Ireland. Because orders ship from within Ireland, there are no customs or import charges. Vitamin K2 is also sold by Irish pharmacies and health stores. All are food supplements regulated under FSAI guidelines with 13.5% VAT.

Related guides

This guide is for educational purposes only and does not constitute medical advice. Healthy Origins Vitamin K2 MK-7, Jarrow Formulas Vitamin K2 MK-7, NOW Vitamin D3 & K2 and Life Extension Vitamins D and K with Sea-Iodine are food supplements regulated under FSAI guidelines and are not medicines; they are not intended to diagnose, treat, cure or prevent any disease or medical condition. If you take anticoagulant medication, have a thyroid condition, high blood calcium, or persistent or worsening symptoms, consult your GP or pharmacist before use. Probiotic.ie is regulated under FSAI food supplement guidelines.

Sources

1. Knapen MH, Drummen NE, Smit E, Vermeer C, Theuwissen E. Three-year low-dose menaquinone-7 supplementation helps decrease bone loss in healthy postmenopausal women. Osteoporos Int. 2013;24(9):2499-2507. PMID 23525894. — pubmed.ncbi.nlm.nih.gov/23525894
2. Knapen MH, Braam LA, Drummen NE, Bekers O, Hoeks AP, Vermeer C. Menaquinone-7 supplementation improves arterial stiffness in healthy postmenopausal women: a double-blind randomised clinical trial. Thromb Haemost. 2015;113(5):1135-1144. PMID 25694037. — pubmed.ncbi.nlm.nih.gov/25694037
3. Diederichsen ACP, Lindholt JS, Möller S, et al. Vitamin K2 and D in patients with aortic valve calcification: a randomized double-blinded clinical trial. Circulation. 2022;145(18):1387-1397. PMID 35465686. — pubmed.ncbi.nlm.nih.gov/35465686
4. Emaus N, Gjesdal CG, Almås B, et al. Vitamin K2 supplementation does not influence bone loss in early menopausal women: a randomised double-blind placebo-controlled trial. Osteoporos Int. 2010;21(10):1731-1740. PMID 19937427. — pubmed.ncbi.nlm.nih.gov/19937427
5. Schurgers LJ, Teunissen KJ, Hamulyák K, Knapen MH, Vik H, Vermeer C. Vitamin K-containing dietary supplements: comparison of synthetic vitamin K1 and natto-derived menaquinone-7. Blood. 2007;109(8):3279-3283. PMID 17158229. — pubmed.ncbi.nlm.nih.gov/17158229
6. Sato T, et al. Comparison of menaquinone-4 and menaquinone-7 bioavailability in healthy women. Nutr J. 2012;11:93. PMID 23140417. — pubmed.ncbi.nlm.nih.gov/23140417
7. Theuwissen E, Cranenburg EC, Knapen MH, et al. Low-dose menaquinone-7 supplementation improved extra-hepatic vitamin K status, but had no effect on thrombin generation in healthy subjects. Br J Nutr. 2012;108(9):1652-1657. PMID 22289649. — pubmed.ncbi.nlm.nih.gov/22289649
8. Geleijnse JM, et al. Dietary intake of menaquinone is associated with a reduced risk of coronary heart disease: the Rotterdam Study. J Nutr. 2004;134(11):3100-3105. PMID 15514282. — pubmed.ncbi.nlm.nih.gov/15514282
9. Beulens JW, et al. High dietary menaquinone intake is associated with reduced coronary calcification. Atherosclerosis. 2009;203(2):489-493. PMID 18722618. — pubmed.ncbi.nlm.nih.gov/18722618
10. Laird E, et al. The prevalence of vitamin D deficiency and the determinants of 25(OH)D concentration in older Irish adults: data from The Irish Longitudinal Study on Ageing (TILDA). J Gerontol A Biol Sci Med Sci. 2018;73(4):519-525. PMID 28958047. — pubmed.ncbi.nlm.nih.gov/28958047
11. EFSA Panel on Nutrition, Novel Foods and Food Allergens. Scientific opinion on the tolerable upper intake level for vitamin D, including the derivation of a conversion factor for calcidiol monohydrate. EFSA Journal. 2023;21(8):8145. — doi.org/10.2903/j.efsa.2023.8145
12. EFSA Panel on Dietetic Products, Nutrition and Allergies. Dietary reference values for vitamin K. EFSA Journal. 2017;15(5):4780. — doi.org/10.2903/j.efsa.2017.4780
13. Health Service Executive (HSE). Vitamin D. — www2.hse.ie/conditions/vitamins-and-minerals/vitamin-d
14. Health Service Executive (HSE). Vitamin K. — www2.hse.ie/conditions/vitamins-and-minerals/vitamin-k
15. Commission Regulation (EU) No 432/2012 establishing a list of permitted health claims made on foods, other than those referring to the reduction of disease risk and to children's development and health. — eur-lex.europa.eu CELEX:32012R0432
16. Revenue Commissioners. Tax and Duty Manual: Food supplement products (VAT treatment). — revenue.ie food supplement products TDM
17. Food Safety Authority of Ireland (FSAI). Food supplements legislation. — fsai.ie/legislation/food-legislation/food-supplements
18. Rønn SH, Harsløf T, Oei L, et al. The effect of vitamin MK-7 on bone mineral density and microarchitecture in postmenopausal women with osteopenia, a 3-year randomized, placebo-controlled clinical trial. Osteoporos Int. 2021;32:185-191. PMID 33030563. — pubmed.ncbi.nlm.nih.gov/33030563
19. Cockayne S, et al. Vitamin K and the prevention of fractures: systematic review and meta-analysis of randomized controlled trials. Arch Intern Med. 2006;166(12):1256-1261. PMID 16801507. — pubmed.ncbi.nlm.nih.gov/16801507