Can you take vitamin K with warfarin?

No, and the reason is unusually clean: it is not a side effect or an interaction in the loose sense. Warfarin works by interfering with vitamin K. A supplement is pushing directly against the mechanism the medicine relies on.

NHS guidance puts it without qualification: “Do not take the herbal remedy St John’s wort or vitamin K supplements while you’re taking warfarin.”

Food is a different question

The instinct on reading that is to stop eating spinach. That instinct is wrong and the guidance says so.

Foods high in vitamin K “can affect how warfarin works”, and then: “You can still eat these foods, but talk to your warfarin clinic”.

The reason is that a warfarin dose is calibrated to what you actually eat. Someone who eats a lot of greens has a dose set for that. What causes trouble is a change: stopping suddenly, starting a green smoothie habit in January, or a fortnight of salads after a fortnight of none.

Consistency, not avoidance. That is the whole rule and it is the opposite of what most people assume.

Where it catches people out

Not in the salad. In the supplement aisle, because Vitamin K is rarely sold on its own.

It appears in multivitamins, in bone health formulas alongside vitamin D and calcium, and increasingly inside vitamin D products because the two are marketed together. Somebody on warfarin buying a bone supplement for an unrelated reason has no prompt to check the panel for it.

Reading the panel on a combination product is the practical defence, and it is the argument of the page on reading labels when you take a prescription.

The other two on the same page

The NHS warfarin guidance names St John's wort in the same sentence, for a different mechanism: it speeds the clearance of medicines, so the drug underperforms.

It also says not to have cranberry juice or cranberry products, and not to drink Grapefruit juice. Three ordinary things in one aisle, and none of them carries a warning about warfarin on its own packaging.