The short answer
- Vitamin D3: worthwhile for people with little sun exposure or low levels; 10–25 mcg (400–1,000 IU) daily is a common maintenance dose; don’t exceed 100 mcg (4,000 IU) a day without medical advice.
- Vitamin K2 (MK-7): usually 45–200 mcg in combination products; benefits for bones and arteries are not proven.
- Warfarin users: avoid K2 supplements unless your clinic approves; they weaken warfarin.
Vitamin D3: who needs it
Vitamin D helps the body absorb calcium and phosphate and supports bones and muscles. Skin makes it in sunlight, so needs depend on latitude, season, skin colour and time outdoors. The NHS advises everyone in the UK to consider 10 micrograms (400 IU) a day in autumn and winter, and all year for people who rarely go outside or cover their skin. Australia, with more sun, recommends supplements mainly for people at risk of deficiency. Higher doses are prescribed to correct a measured deficiency. D3 (cholecalciferol) raises blood levels more effectively than D2.
Vitamin K2 (MK-7): what it does
Vitamin K activates proteins that bind calcium: osteocalcin in bone and matrix Gla protein in blood vessel walls. That’s the basis for pairing it with vitamin D, which increases calcium absorption. K1 (from leafy greens) mainly supports clotting; K2 (menaquinones, from fermented foods such as natto and some cheeses) includes MK-4 and MK-7. MK-7 has a long half-life, which is why it’s used in once-daily supplements.
| Claim |
What the evidence shows |
| K2 prevents fractures |
Mixed; some Japanese trials used high-dose MK-4 (45 mg) with benefit, but MK-7 trials in Western populations show little effect on fractures |
| K2 prevents arterial calcification |
Trials have mostly not shown slowing of calcification |
| Vitamin D causes calcium to deposit in arteries without K2 |
Not supported at normal vitamin D doses |
| D3 + K2 together is safe |
Generally yes at typical doses, except with warfarin |
Choosing a product
- Check the doses per serving: vitamin D in mcg and IU (1 mcg = 40 IU), K2 in mcg and its form (MK-7, “all-trans”).
- Liquid drops suit people who struggle with tablets; check how many drops per dose.
- Vegan products use D3 from lichen and K2 from fermentation; standard D3 comes from lanolin.
- Avoid mega-doses (10,000 IU D3 daily products) unless prescribed.
- Add-ons such as magnesium are fine but raise the price; compare cost per useful dose.
- Quality marks: USP Verified, NSF, or AUST L/AUST R numbers in Australia.
Safety and interactions
Warfarin: vitamin K directly opposes warfarin. Starting, stopping or changing a K2 supplement can push your INR out of range. Newer anticoagulants (apixaban, rivaroxaban, dabigatran) aren’t affected by vitamin K. High vitamin D intake over months can raise blood calcium, causing nausea, thirst, confusion and kidney stones; people with sarcoidosis, kidney stones or high calcium should ask before taking it.
More on vitamin D, other supplements and medicine interactions is in the vitamins and supplements guide.
Frequently asked questions
Should I take vitamin D3 and K2 with food?
Yes. Both are fat-soluble, so take them with a meal that contains some fat.
Can I take D3 + K2 with magnesium?
Yes, there’s no problem combining them. Magnesium is involved in vitamin D metabolism, but extra magnesium is only needed if your diet is low.
Is liquid D3 with K2 better than capsules?
Not necessarily. Absorption is similar when taken with food; choose the form you’ll take consistently, at the right dose.
How much K2 should I take?
Products usually provide 45–200 mcg of MK-7. There’s no established upper limit, but there’s also no proven benefit from high doses.
Sources and further reading
CanforaMedicine Guide
