Warfarin Side Effects and Bleeding Risks: What You Must Know as a Patient

Warfarin has been used for over 70 years to prevent dangerous blood clots. It’s one of the most common blood thinners prescribed in the UK and the US, especially for people with atrial fibrillation, deep vein clots, or mechanical heart valves. But while it works, it’s not without serious risks. Warfarin side effects can be life-threatening - especially bleeding. If you’re taking warfarin, you need to know what to watch for, when to act, and how to stay safe every single day.

What Warfarin Does - and Why It’s Still Used

Warfarin works by blocking vitamin K, which your body needs to make clotting factors. Without those factors, blood doesn’t clot as easily. That’s good if you’re at risk of stroke or a pulmonary embolism. But it’s dangerous if you cut yourself, fall, or develop an internal bleed.

Despite newer blood thinners like apixaban and rivaroxaban, warfarin is still widely used. Why? It’s cheap - generic warfarin costs about £3 to £8 a month in the UK. It’s also reversible. If you start bleeding badly, doctors can give you vitamin K or a concentrated clotting factor to reverse its effects quickly. Newer drugs don’t have that advantage. For some people - like those with mechanical heart valves - warfarin is still the only safe option.

The Biggest Risk: Bleeding

Bleeding is the #1 danger with warfarin. About 1 in 10 people on warfarin will have a major bleed each year. That’s not rare. It’s common enough that every patient needs to treat it like a medical emergency.

Minor bleeding might seem harmless - a nosebleed that lasts longer than 5 minutes, bleeding gums when brushing your teeth, or unexplained bruises. But these are warning signs. They mean your blood is taking too long to clot.

Major bleeding is far more serious. Call 999 or go to A&E if you have:

• Red, pink, or brown urine
• Black, tarry, or bloody stools
• Vomiting blood or material that looks like coffee grounds
• Sudden, severe headache or dizziness
• Weakness on one side of your body
• Severe back or abdominal pain
• Heavy vaginal bleeding outside your period
• Any bleeding that won’t stop after 10 minutes of pressure

These symptoms can mean internal bleeding - in your brain, gut, or around your organs. Brain bleeds are especially deadly. About 0.2% to 0.5% of warfarin users have one each year. The faster you get help, the better your chances.

Your INR Is Your Lifeline

Doctors measure how well warfarin is working with a blood test called INR (International Normalized Ratio). The goal is usually between 2.0 and 3.0. For people with mechanical heart valves, it might be higher - around 2.5 to 3.5.

Here’s the key: every point your INR goes above 3.0 doubles your risk of major bleeding. At INR 4.0, your risk is 4 to 8 times higher than when you’re in range. That’s why regular testing matters.

If your INR is stable, you’ll usually get tested every 4 weeks. But if you’ve just started warfarin, changed your dose, or been sick, you might need testing every week. Some people even test at home with a finger-prick device like the CoaguChek Pro II. Studies show this can improve your time in range by 15% to 20% - meaning fewer bleeds and fewer hospital visits.

What Makes Bleeding Worse

Some risks you can’t change. Age over 65? That doubles your bleeding risk. History of stomach ulcers? That triples it. High blood pressure? That increases risk by almost 3 times. If you have three or more of these risk factors, your chance of a major bleed jumps to nearly 20% per year.

But some risks you can control:

• INR too high - the biggest modifiable risk. Never skip your blood tests.
• Medications - NSAIDs like ibuprofen and naproxen can increase stomach bleeding risk by 2 to 4 times when taken with warfarin. Use paracetamol instead, unless your doctor says otherwise.
• Diet changes - eating a big bowl of spinach or kale one day and none the next can swing your INR by 0.5 to 1.0 points. Keep your vitamin K intake steady. Aim for 60-80 mcg daily. That means consistent portions of leafy greens - not none, not tons.
• Alcohol - heavy drinking can make your INR go up and down unpredictably. Stick to moderate amounts, if any.
• Supplements - garlic, ginkgo, fish oil, and even some herbal teas can thin your blood further. Always tell your doctor what you’re taking.

Rare but Serious Side Effects

Bleeding isn’t the only problem. Warfarin can cause rare but scary side effects.

Warfarin-induced skin necrosis - This happens in about 1 in 10,000 people. Within days of starting warfarin, you might see dark, painful patches of skin, usually on the thighs, breasts, or buttocks. It’s linked to a sudden drop in protein C, a natural anticoagulant. If you see this, stop the drug and get to a hospital immediately.

Purple toe syndrome - A few weeks after starting warfarin, your toes might turn purple or blue. It’s caused by tiny cholesterol crystals blocking blood vessels. It’s not life-threatening, but it’s painful and needs medical attention.

Calciphylaxis - This is extremely rare, mostly in people with kidney failure. Calcium builds up in small blood vessels, causing painful skin ulcers. If you have kidney disease and notice hard, painful spots on your legs, tell your doctor right away.

How to Stay Safe Every Day

Living on warfarin isn’t about fear - it’s about smart habits.

• Use an electric razor, not a blade. Blades cause small cuts you won’t notice - but they can bleed longer.
• Use a soft-bristle toothbrush and waxed floss. Don’t brush hard. Gum bleeding is common - but if it doesn’t stop, call your doctor.
• Avoid contact sports. No rugby, football, boxing, or skiing without proper protection. Even biking carries fall risk.
• Wear a medical alert bracelet or carry a card that says you’re on warfarin. In an emergency, that saves lives.
• Always tell any doctor, dentist, or pharmacist you’re on warfarin - even for a simple filling. Some procedures need temporary adjustments.
• Keep a log of your INR results, doses, and any symptoms. It helps your doctor spot patterns.

What to Do If You Bleed

Minor bleeding - like a small nosebleed that stops in under 5 minutes - usually doesn’t need emergency care. But call your anticoagulation clinic. They might want to check your INR.

For serious bleeding, don’t wait. Call 999. Don’t try to drive yourself. Emergency teams will:

• Test your INR right away
• Stop your warfarin
• Give you vitamin K (by injection) to reverse the effect
• Use prothrombin complex concentrate (PCC) if bleeding is life-threatening - this works in minutes

If you have a brain bleed, reversal must happen within 30 to 60 minutes. Every minute counts.

When to Consider Switching

Newer blood thinners - called DOACs - are now first-choice for most people with atrial fibrillation or clots. They don’t need regular INR checks, have fewer food interactions, and carry about 30% less risk of major bleeding than warfarin.

But they’re not for everyone. If you have a mechanical heart valve, severe kidney disease, or are pregnant, warfarin is still the best option. Your doctor should review your situation every year. Ask: “Is warfarin still the right choice for me?”

Bottom Line: Stay Informed, Stay Vigilant

Warfarin saves lives - but it can also end them if not managed carefully. The good news? You’re not helpless. You have control. Test your INR. Eat consistently. Avoid risky meds. Watch for bleeding. Speak up.

Most people on warfarin live full, active lives - as long as they stay informed and follow their plan. Don’t ignore the small signs. Don’t skip the tests. Don’t assume it’s fine because you feel okay. Bleeding doesn’t always come with pain. Sometimes, it just comes with a bruise, a nosebleed, or a change in your urine.

Know the signs. Know your numbers. And never, ever stop taking warfarin without talking to your doctor first.