Vitamin K

Vitamin K rarely attracts the same attention as vitamin C or vitamin D, yet questions about it come up regularly in UK pharmacies. Customers often ask whether they should take a supplement, whether they are getting enough from food, or whether vitamin K could interfere with prescribed medicines.

This guide explains what vitamin K does in the body, who might benefit from supplementation, and when professional advice is essential. The focus is on clear, practical information that helps pharmacy customers make informed decisions.

What Is Vitamin K and What Does It Do?

Vitamin K is a fat-soluble vitamin with two core functions: helping blood clot properly and supporting bone health.

Its best-known role is in normal blood clotting. Vitamin K is required for the liver to produce several clotting factors that prevent excessive bleeding. Without enough vitamin K, even small cuts or injuries could bleed for longer than expected. This function is so central that the vitamin takes its name from the German word Koagulation.

Vitamin K also plays a supportive role in bone health. It activates proteins that help guide calcium into bones and teeth, where it is needed for strength, rather than allowing it to accumulate in soft tissues. Although vitamin K is stored in the liver, these stores are relatively small and are used up quickly compared with some other fat-soluble vitamins.

Vitamin K1 vs Vitamin K2: What’s the Difference?

Vitamin K exists in two main forms: vitamin K1 and vitamin K2. While closely related, they come from different sources and behave slightly differently in the body.

Vitamin K1 comes mainly from plant foods, especially green leafy vegetables. It accounts for most of the vitamin K in the typical UK diet and is primarily involved in blood clotting.
Vitamin K2 refers to a group of compounds found in animal foods, fermented products, and produced in small amounts by gut bacteria. It stays in the bloodstream for longer and can reach tissues such as bones more easily.

This difference explains why vitamin K2 is often highlighted in bone health supplements. Some research suggests it may be more effective than K1 at activating proteins involved in bone metabolism. However, both forms contribute to overall vitamin K status, and the body can convert small amounts of K1 into K2, although this process is limited.

Food Sources of Vitamin K

Most people obtain enough vitamin K through everyday foods, often without realising it.

Vitamin K1 is especially abundant in green vegetables such as spinach, kale, broccoli, Brussels sprouts, cabbage, and lettuce. Vegetable oils, including rapeseed and soya oil, also make a meaningful contribution. A single serving of cooked leafy greens can easily provide more than the daily recommended intake.

Vitamin K2 appears in smaller amounts in the UK diet but is found in foods such as cheese, eggs, chicken, and beef. Hard cheeses tend to contain more K2 than soft varieties. Fermented foods are the richest sources, although foods like natto are not commonly eaten in the UK.

Because vitamin K is fat-soluble, it is absorbed more effectively when eaten with fat. Cooking vegetables with oil or adding dressing to salads helps improve absorption. While vitamin K1 from plants is absorbed less efficiently than K2 from animal or fermented foods, regular vegetable intake still provides ample amounts for most people.

Who Might Benefit from a Vitamin K Supplement?

For most healthy adults, vitamin K supplements are unnecessary. Daily requirements are modest, and a normal diet containing vegetables easily meets them.

Some people may have a higher risk of low vitamin K intake or absorption, including:

People with conditions that affect fat absorption, such as coeliac disease, Crohn’s disease, or chronic pancreatitis
Those following very restricted diets that exclude vegetables or animal products
People taking long-term antibiotics, which can disrupt gut bacteria involved in vitamin K2 production

Older adults sometimes consider vitamin K2 supplements for bone health. Some studies suggest that long-term use of vitamin K2 may help support bone mineral density, particularly in postmenopausal women. However, evidence is mixed, and vitamin K supplements should not be viewed as a standalone treatment for osteoporosis.

Before starting any supplement, particularly for bone health, it is sensible to speak with a pharmacist or GP. They can help assess whether supplementation is appropriate and ensure it does not interfere with existing medicines or health conditions.

Vitamin K and Medications: Important Safety Information

Vitamin K has a well-established interaction with warfarin and other vitamin K antagonist anticoagulants.

Warfarin works by interfering with vitamin K’s role in blood clotting. Sudden changes in vitamin K intake—whether from food or supplements—can make warfarin harder to manage and affect INR levels.

For people taking warfarin, the key points are:

Do not avoid vitamin K entirely — this can destabilise anticoagulation
Keep intake consistent, particularly from green vegetables
Avoid starting supplements without medical advice

Any planned dietary changes or supplement use should always be discussed with an anticoagulation clinic, pharmacist, or GP first.

Other medicines can also influence vitamin K status. Some antibiotics may lower vitamin K levels, while certain anticonvulsants can interfere with vitamin K metabolism. Pharmacists can advise on individual medicines and potential interactions at the point of dispensing.

Is Vitamin K Deficiency Common in the UK?

True vitamin K deficiency is rare in healthy UK adults. Most people consume enough through their diet, and symptoms such as excessive bruising or prolonged bleeding are uncommon.

Newborn babies are a notable exception. Babies are born with very low vitamin K levels, which is why all newborns in the UK are routinely offered vitamin K shortly after birth. This prevents vitamin K deficiency bleeding, a rare but potentially serious condition.

In adults, low vitamin K status is more likely in people with malabsorption disorders, highly restricted diets, or prolonged antibiotic use. Some researchers suggest that vitamin K intake sufficient for blood clotting may not always be optimal for bone health, but this remains an area of ongoing research rather than settled guidance.

Pharmacist Comment

"Most people don’t need a vitamin K supplement if they eat a balanced diet that includes vegetables. Foods like broccoli, spinach, and cabbage usually provide more than enough for normal blood clotting and bone health.
The main area of concern is for people taking warfarin. For these patients, consistency matters far more than avoiding vitamin K altogether. Any changes to diet or supplements should always be discussed first.
If someone has absorption issues, has been on long-term antibiotics, or is worried about bone health, a pharmacist or GP can help decide whether supplementation is appropriate."
Alessandro Grenci, Superintendent Pharmacist

Final Thoughts

Vitamin K plays a vital role in blood clotting and contributes to bone health throughout life. While it may receive less attention than other vitamins, its importance should not be underestimated.

For most people in the UK, a varied diet that includes green vegetables provides all the vitamin K needed. Supplements may be useful in specific situations, but they are not necessary for the majority of adults. Anyone taking anticoagulant medication should approach vitamin K with particular care and seek professional advice before making changes.

When questions arise about diet, supplements, or medicines, pharmacists and GPs are well placed to offer clear, personalised guidance.