What Is INR and Why Does It Matter?
INR (International Normalised Ratio) measures how long your blood takes to clot compared to average. A normal INR is 1.0. On warfarin, the target is usually 2.0–3.0 — meaning your blood takes 2–3 times longer to clot than normal.
| Condition | INR Target |
|---|---|
| Atrial fibrillation (AF) | 2.0–3.0 |
| DVT / pulmonary embolism (first episode) | 2.0–3.0 |
| Recurrent DVT / PE | 2.0–3.0 (or 2.5–3.5) |
| Mechanical heart valve (aortic) | 2.0–3.0 |
| Mechanical heart valve (mitral) | 2.5–3.5 |
Danger ZonesINR >5.0: High bleeding risk — contact your anticoagulation clinic immediately. INR <1.5: Blood clot risk too high — dose increase needed urgently. Never adjust your own warfarin dose without guidance.
Foods That Affect Your INR
| Food (high vitamin K) | Effect on INR | Advice |
|---|---|---|
| Kale, spinach, Brussels sprouts | Lowers INR (reduces warfarin effect) | Eat consistently — don't avoid, just keep intake steady |
| Cranberry juice / products | Raises INR (increases bleeding risk) | Avoid large amounts |
| Grapefruit | Can raise INR | Limit intake |
| Alcohol (heavy) | Raises INR significantly | Limit to 1–2 units/day max |
| Green tea (large amounts) | Lowers INR | Keep intake consistent |
Consistency KeyDon't eliminate vitamin K foods — just eat them consistently. A sudden change (e.g. starting a healthy diet rich in green vegetables) will throw your INR off. Warn your anticoagulation clinic if you make major diet changes.
Common Drug Interactions
| Drug | Effect on INR |
|---|---|
| Aspirin, NSAIDs | Increases bleeding risk (different mechanism) |
| Antibiotics (most) | Can raise INR — check INR 5–7 days after starting |
| Amiodarone | Dramatically raises INR |
| Antifungals (fluconazole) | Significantly raises INR |
| Carbamazepine, rifampicin | Lowers INR |
| St John's Wort | Lowers INR significantly |
What should I do if I miss a dose?
Take the missed dose on the same day if you remember. Never double dose. Check your INR sooner if you miss multiple doses.
I'm bleeding (nose, gums, bruising) — is this from warfarin?
Minor bleeding is common on warfarin. However, seek emergency help for: coughing or vomiting blood, blood in urine or stools, severe headache, prolonged or heavy bleeding.
Can I have surgery on warfarin?
Usually warfarin is stopped 5 days before surgery. Your surgeon and anticoagulation team will coordinate. Some low-risk procedures (e.g. dental extraction) can be done with INR <2.5.
Are newer blood thinners (DOACs) better than warfarin?
For most conditions, DOACs (apixaban, rivaroxaban, dabigatran) are as effective with fewer dietary interactions and no INR monitoring needed. Ask your doctor if you're a candidate to switch.
Medical Disclaimer: This page is for general education only and does not replace professional medical advice. Always consult a qualified healthcare provider.