Walk down any supplement aisle in October and you'll find D3 on one shelf and D3 with K2 on the next, usually a few pounds dearer and wrapped in the language of synergy. Fair question: is the K2 doing something, or is it a tidy way to charge more for the same sunshine vitamin?
Short version: the biology makes sense, the human evidence is thinner than the marketing, and the thing that matters most is getting your D3 dose right in the first place.
Why the two get paired
The NHS says everyone should consider a daily vitamin D supplement of 10 micrograms from autumn through winter, when the sun isn't strong enough for your skin to make it.[1] Vitamin D has authorised health claims for contributing to the normal absorption and use of calcium and phosphorus, the maintenance of normal bones, and normal muscle and immune function. Vitamin K is authorised for contributing to normal blood clotting and the maintenance of normal bones.[2] The NHS puts adult need at roughly 1 microgram per kilogram of body weight a day, mostly from leafy greens, vegetable oils and cereal grains.[3]
The case for combining them goes like this. D3 helps you absorb calcium; vitamin K helps activate proteins, osteocalcin among them, involved in getting calcium into bone. It's a plausible mechanism. Plausible isn't proven, and the review we lean on most here says exactly that.[4]
What the trials actually found
The best-known K2 study followed 244 postmenopausal women aged 55 to 65 for three years. Those taking 180 micrograms of MK-7 a day lost less lumbar spine bone density than those on placebo, roughly 0.5% against 1.5%, as reported in Osteoporosis International.[5] Worth knowing: it tested K2 alone, and the manufacturer of the MK-7 funded it. It doesn't tell us what happens when you add K2 to a D3 you're already taking.
On the combination itself, a 2020 meta-analysis of eight randomised trials (971 people, mostly women) found moderate-quality evidence that vitamin D plus vitamin K can improve bone mineral density, with K2 looking stronger than K1.[6] Then the caveats. The trials are small and short, they measure density rather than fractures, and the benefit seems to sit with postmenopausal women who already have low bone mass. No large trial has shown fewer fractures in the general population.[4]
So do you actually need K2?
Not in the way you need D3 in January. If you eat greens most days you're already getting vitamin K, and K2 is an option, not a requirement. It's most worth a conversation with your GP if bone health is a particular concern for you, for instance after the menopause.
One firm caution: if you take warfarin or another vitamin K antagonist, don't add K2 without speaking to your doctor or pharmacist. Vitamin K can blunt those drugs, which is why people on them are told to keep their intake steady.[4]
What to check on the label
- Micrograms of D3 per serving — and whether the serving is one tablet or two. Compare it with 10 micrograms, not with the biggest number on the pack.
- The form of K — MK-7 is the K2 used in the trial research above; K1 is the plant form.
- Total K in micrograms — the NHS says 1mg or less a day from supplements is unlikely to cause harm.[3]
Where our own tablet sits
Strength is where D3 and K2 products differ most, so here are our numbers. Our Vitamin D3 & K2 Tablets give 4,000iu (100 micrograms) of D3 and 150 micrograms of K2 as MK-7 across a two-tablet daily serving, with coconut MCTs. That D3 figure is the NHS upper limit, not a starting point.[1] It's a high-strength option for people whose GP or blood test points that way, and you shouldn't stack it with another vitamin D supplement. For everyone else, 10 micrograms is the number to aim for, and we've written that up in Vitamin D3 Dosage UK: How Much Do You Actually Need?.
If a higher-strength D3 with K2 is what you and your GP have landed on, this is the one built to those label numbers:
The gold standard is boring
Know your D3 dose, decide whether K2 earns a place next to it, and be honest about what the trials do and don't show. That's the whole routine. Get the D right first; the K2 is a refinement.
Frequently asked questions
Can you take vitamin K2 and D3 together?
Yes, and they're commonly sold together. The one real exception is anyone taking warfarin or a similar vitamin K antagonist, because vitamin K can reduce how well those medicines work. If that's you, speak to your GP or pharmacist before adding K2.
How long does vitamin D3 with K2 take to work?
Bone is slow tissue. The trials behind the bone-density findings ran from six months to three years, so think in seasons rather than weeks. There's no quick-result version of this story, and no supplement should replace a conversation with your GP if you have concerns about your bones.
How much vitamin D3 should I take with K2?
The NHS suggests 10 micrograms (400 IU) a day for adults through autumn and winter, and says not to go above 100 micrograms (4,000 IU) a day. Most people sit nearer the first number. Our full breakdown is in Vitamin D3 Dosage UK: How Much Do You Actually Need?.
Is it safe to take vitamin K2 every day?
The NHS says 1mg or less of vitamin K a day from supplements is unlikely to cause harm, though it also says there isn't enough evidence on higher doses. Check with your GP first if you're on blood-thinning medication or have a health condition.
Sources
- NHS — vitamin D: 10 micrograms a day through autumn and winter, and the 100 microgram upper limit. https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/
- Commission Regulation (EU) No 432/2012 — list of permitted health claims, including the vitamin D and vitamin K claims referenced. https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:32012R0432
- NHS — vitamin K: adult need, food sources and the 1mg supplement guidance. https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-k/
- Science-Based Medicine (Gavura, June 2025) — review of vitamin D plus K co-supplementation, plausibility of the mechanism, limits of the trial evidence and the warfarin caution. https://sciencebasedmedicine.org/should-you-take-vitamin-k-with-your-vitamin-d/
- Knapen et al., Osteoporosis International (2013) — three-year MK-7 trial in 244 postmenopausal women, as reported by NutraIngredients (manufacturer-funded). https://www.nutraingredients.com/Article/2013/03/27/Vitamin-K2-shows-extremely-important-improvements-in-clinical-outcomes-for-bone-health/
- Examine.com summary (July 2020) — meta-analysis of 8 randomised trials, 971 participants, on combined vitamin D and K and bone mineral density. https://examine.com/summaries/study/g983b0/
At Oxford Origins, your wellbeing is our priority, and although we pride ourselves on our expertise in nutrition and supplementation, it's important to acknowledge the individuality of each person. Articles published by Oxford Origins are not intended to treat, diagnose, cure or prevent any disease, or replace the advice of your GP. We always recommend consulting with a healthcare provider if you encounter any health concerns, and we'll always be here to support you so you're never alone on your journey.
