Nursing care
Warfarin: what to check before you give it
Written and reviewed by Dana Whitfield, RN, MSN · 4 min read · Updated September 2026
Short answer
Warfarin blocks vitamin K-dependent clotting factor synthesis, thinning the blood to prevent clot formation. Most indications target an INR of 2 to 3, and vitamin K is the reversal agent for bleeding or excessive anticoagulation. Patients should eat a consistent amount of vitamin K-rich vegetables rather than avoiding them altogether.
Mechanism, simply
Warfarin inhibits vitamin K epoxide reductase, the enzyme the liver needs to activate clotting factors II, VII, IX, and X. Without active vitamin K, these factors are made but cannot function, so the clotting cascade slows and the blood takes longer to clot. Because existing clotting factors have to be depleted first, the anticoagulant effect takes several days to become fully established, which is why warfarin is usually started alongside a faster-acting agent like heparin during initiation.
This delayed onset also means the effect persists for several days after the drug is stopped, since new, functional clotting factors have to be resynthesised. Understanding this lag is central to interpreting INR trends and to planning around invasive procedures.
Indications you will see on the ward
Warfarin is prescribed for atrial fibrillation to prevent embolic stroke, for mechanical heart valves, and for treatment or prevention of venous thromboembolism including deep vein thrombosis and pulmonary embolism. Mechanical valve patients typically need a higher target INR, often 2.5 to 3.5, than the standard 2 to 3 used for most other indications, so always check the specific target ordered for that patient rather than assuming.
It remains in wide use despite newer direct oral anticoagulants because it is the required choice for mechanical heart valves, where the DOACs are not effective, and because it is reversible, inexpensive, and familiar in settings with limited monitoring resources.
Assessment before administration
Check the most recent INR before every dose and hold the dose, notifying the prescriber, if the result is outside the ordered parameters or significantly above target. Assess for any signs of bleeding, including bruising, gum bleeding, blood in urine or stool, and ask about any new medications, supplements, or dietary changes since the last visit, since so many substances alter warfarin's effect.
Review the patient's full medication list for interacting drugs at every encounter, not just at initiation, since warfarin's narrow therapeutic index means even a single new prescription can push the INR out of range. Confirm the patient's target INR range before adjusting expectations of what counts as therapeutic for them specifically.
Toxicity and the antidote
Warfarin toxicity presents as bleeding: easy bruising, epistaxis, haematuria, melena, or in severe cases intracranial haemorrhage. An INR far above the therapeutic range without active bleeding is managed by holding doses and sometimes giving oral vitamin K; active or severe bleeding requires more urgent reversal.
Vitamin K (phytonadione) is the specific antidote, restoring the liver's ability to activate clotting factors, though its full effect takes hours to a day or more because new factors still need to be synthesised. For serious bleeding, prothrombin complex concentrate or fresh frozen plasma is given alongside vitamin K to replace clotting factors immediately, since vitamin K alone acts too slowly for an emergency.
Interactions that matter
Many antibiotics, including metronidazole and fluoroquinolones, potentiate warfarin's effect and raise INR by disrupting gut flora that produce vitamin K or by inhibiting warfarin metabolism. NSAIDs and aspirin increase bleeding risk through an additive antiplatelet effect even without changing the INR itself, so their use alongside warfarin needs careful review.
Alcohol, amiodarone, and many antifungals also raise INR, while inducers such as rifampin, carbamazepine, and a high, sudden intake of vitamin K-rich food can lower it. The theme across all of these is variability: any new medication, supplement, or significant dietary change is a reason to recheck the INR sooner rather than waiting for the next scheduled draw.
What the patient must be told
Teach the patient to eat a consistent amount of vitamin K-containing vegetables, such as leafy greens, week to week, rather than cutting them out, since a stable intake keeps the INR predictable and cutting greens abruptly then reintroducing them causes swings that are harder to manage than steady moderate intake.
Tell the patient to report any unusual bruising, bleeding gums, blood in urine or stool, or prolonged bleeding from a cut, and to inform every prescriber and pharmacist that they take warfarin before starting any new medication, including over-the-counter products and herbal supplements. Reinforce the need for regular INR monitoring, consistent dosing at the same time each day, and the use of a soft toothbrush and electric razor to reduce bleeding risk from minor trauma.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our cardiovascular practice questions are the closest set to what this page covers.
Common questions
What is the target INR for warfarin?
For most indications, including atrial fibrillation and venous thromboembolism, the target is 2 to 3. Patients with a mechanical heart valve usually need a higher target, often 2.5 to 3.5, so always confirm the specific range ordered.
Should patients on warfarin avoid green vegetables?
No. They should eat a consistent amount of vitamin K-rich vegetables from week to week rather than avoiding them. Sudden changes, in either direction, are what destabilise the INR, not steady intake.
What reverses warfarin in an emergency?
Vitamin K is the specific antidote but acts over hours. For serious or life-threatening bleeding, prothrombin complex concentrate or fresh frozen plasma is given alongside vitamin K to replace clotting factors immediately.
Why does warfarin take days to start working?
It blocks activation of new clotting factors but does not remove the ones already circulating. Those existing factors have to be depleted through normal turnover before the anticoagulant effect is fully established, which typically takes several days.
Can warfarin and aspirin be taken together?
They can be combined in specific clinical situations, but the combination increases bleeding risk through an additive antiplatelet effect, even when the INR is within target range, so it requires careful prescriber review.
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