Warfarin Toxicity (High INR)

What INR means

Warfarin blocks vitamin K-dependent clotting factors made in the liver. The international normalized ratio (INR) measures how long blood takes to clot compared with normal. An INR of 1 is normal without warfarin. An INR of 2 means blood takes about twice as long to clot.

Bleeding risk rises sharply above an INR of about 4 to 5.

Causes of warfarin toxicity

Medicine interactions (most common)

Many drugs increase warfarin's effect, often within days:

• Antibiotics: trimethoprim-sulfamethoxazole, metronidazole, fluconazole and other azole antifungals, ciprofloxacin, levofloxacin, clarithromycin, erythromycin
• Heart medicines: amiodarone (a strong and long-lasting effect), some statins
• Pain relievers: high doses of acetaminophen over several days; NSAIDs and aspirin increase bleeding risk without raising INR
• Supplements: fish oil, high-dose vitamin E, ginkgo, garlic supplements, cranberry in large amounts, turmeric and dong quai
• Thyroid hormone changes

Always ask your anticoagulation clinic or pharmacist to check any new medicine, even a short course of antibiotics.

Diet and lifestyle

• Eating less vitamin K than usual: fewer leafy greens such as spinach, kale, collards and broccoli, or poor appetite
• Binge drinking alcohol
• Nutritional shakes or diets that change vitamin K intake

Illness

• fever, diarrhea or vomiting
• liver disease or heart failure flare (reduced clotting factor production)
• overactive thyroid
• cancer

Dose errors

• taking an extra tablet, doubling up after a missed dose, confusing tablet strengths (warfarin tablets come in many colors and strengths)
• recent dose increases

Accidental ingestion

Long-acting "superwarfarin" rat poisons (brodifacoum) cause prolonged severe anticoagulation lasting weeks to months. Children and pets who swallow them need urgent assessment. Some synthetic cannabinoid products have been contaminated with brodifacoum.

Symptoms

A high INR often causes no symptoms until bleeding happens, which is why regular INR checks matter. Signs include:

• easy or unexplained bruising
• nosebleeds or bleeding gums
• prolonged bleeding from cuts
• heavier periods
• pink, red or brown urine (blood in the urine)
• red or black stools, a sign of gastrointestinal bleeding
• coughing or vomiting blood
• headache, dizziness, weakness or fatigue, which with confusion can mean bleeding in the brain

See anticoagulant-associated bleeding for how major bleeding on any blood thinner is handled.

How high INR is managed

Management depends on how high the INR is and whether you are bleeding. These are decisions for your doctor or anticoagulation clinic, but typical approaches based on CHEST guidance are:

Fresh frozen plasma is used if PCC is not available. After the episode, the cause is identified and the warfarin dose adjusted. Some people are switched to a DOAC such as apixaban, which does not need INR monitoring and has fewer food interactions, if suitable for their condition. DOACs are not used for mechanical heart valves. For people taking warfarin for atrial fibrillation, a DOAC is often the preferred option.

Preventing warfarin toxicity

• Take warfarin at the same time every day, exactly as prescribed. Use a pill organizer and a log.
• If you miss a dose, follow your clinic's instructions; do not double the next dose.
• Keep vitamin K intake consistent. You do not need to avoid greens, but eat similar amounts each week.
• Limit alcohol to no more than 1 to 2 drinks a day and avoid binges.
• Tell every prescriber and pharmacist that you take warfarin. Get your INR checked within 3 to 5 days of starting or stopping any interacting medicine.
• Avoid NSAIDs and aspirin unless your doctor prescribes them.
• Report illness, especially vomiting, diarrhea or not eating.
• Consider home INR testing if your doctor agrees.
• Store warfarin away from children and pets.

Frequently asked questions

What are the symptoms of warfarin toxicity?

Often there are no symptoms until bleeding occurs. Signs include easy bruising, nosebleeds, bleeding gums, prolonged bleeding from cuts, pink or brown urine, red or black stools and heavy periods. Severe headache, confusion or weakness can mean bleeding in the brain and is an emergency.

What should I do if my INR is high?

Contact your anticoagulation clinic or doctor the same day for advice. For a mildly high INR without bleeding, they usually lower or skip a dose and recheck sooner. If you are bleeding, have a severe headache, or your INR is very high, seek urgent care.

What causes INR to go up suddenly?

The most common causes are new medicines, especially antibiotics, antifungals and amiodarone, eating less vitamin K, alcohol binges, illness with fever, vomiting or diarrhea, and dosing mistakes. Liver problems and heart failure can also raise INR.

Should I stop eating greens on warfarin?

No. Vitamin K in leafy greens is part of a healthy diet. The key is consistency: eat similar amounts each week so your warfarin dose can be set to match. Sudden increases lower your INR, and sudden decreases raise it.

How is warfarin overdose treated?

Treatment depends on the INR and bleeding. Mild excess is managed by holding doses. Very high INR is treated with vitamin K. Serious bleeding is treated in the hospital with intravenous vitamin K and prothrombin complex concentrate, which reverses warfarin within minutes.