The Side Effects of Blood Thinners

Blood thinners do a good job of treating clots. They also work to stop them from happening. But these drugs can cause serious side effects. Bleeding inside or outside of your body is the most common. Here’s what you need to know.

Wafarin, Heparin, and Low Molecular Weight Heparin

These are the three most commonly used blood thinners. Certain factors can keep them from working the right way, including:

  • The wrong dose
  • Bad interactions with other drugs, vitamins, or supplements
  • Changes in what you eat or drink
  • Other medical conditions

When this happens, your blood clots at a much slower rate. It might be hard to stop a nosebleed or cut on your finger. Or, the bleeding may happen inside. For instance, if you fall and hit your head, you could have bleeding in your brain.

Another serious, but less common, side effect of warfarin is necrosis. That’s when your skin starts to die.

You may lose bone strength if you take heparin for a long time. This raises your chances of breaking a bone.

You give low molecular weight heparin to yourself as an injection, or shot. You may have redness or a bruise where you put the needle in. Your liver enzymes may also be higher. But you’re less likely to lose bone strength. And researchers don’t know of any bad interactions with food or drink.


How to Manage Side Effects

Get your blood checked often if you’re on warfarin or heparin. This is an international normalized ration (INR) test. It tells you how long it takes your blood to clot.

You can also try to stop uncontrollable bleeding before it starts. Here are a few tips:

  • Use a soft toothbrush and waxed dental floss
  • Use an electric razor
  • Don’t trim corns or calluses
  • Wear gloves when you do yardwork or use sharp tools
  • Wear a helmet when bike riding
  • Do low-impact sports like walking or swimming

Other tips include:

Pay attention to how much vitamin K you’re getting. It helps your blood clot. But too much can stop warfarin from working. Talk to your doctor to find the right balance.

Don’t drink too much alcohol. This raises your risk of bleeding. One to two drinks a day is a safe amount.

Tell people you’re on a blood thinner. Wear a medical alert bracelet or carry a card with your medications on it. Make sure your doctor, dentist, and other health care providers know. Tell them if you also take herbal supplements or vitamins.

When to Call Your Doctor

Call your doctor right away if you have these side effects while taking blood thinners:

  • Bleeding during your period that’s heavier than normal
  • Red or brown pee
  • Poops that’s red or looks like tar
  • Nose or gum bleeding that doesn’t stop quickly
  • Brown or bright red vomit
  • A severe headache or stomachache
  • Unusual bruising
  • A cut that won’t stop bleeding
  • Dizziness or weakness

You should also call them if you cough up anything red-colored or fall and bump your head.

Call your doctor right away if you have any of these symptoms. You may have a blood clot. They include:

  • Blurred vision
  • Chest pain
  • Severe weakness or dizziness
  • Tingly hands, feet, or face
  • Can’t move
  • Swollen or red arms, calves, or feet
WebMD Medical Reference Reviewed by Brunilda Nazario, MD on February 01, 2021



Agency for Healthcare Research and Quality: “Blood Thinner Pills: Your Guide to Using Them Safely.”

Children’s Minnesota: “Anticoagulants: Treatment of Blood Clots.”

Mayo Clinic: “Warfarin side effects: Watch for interactions.”

International Journal of Molecular Sciences: “Anticoagulants and Osteoporosis.”

National Blood Clot Alliance: “Low Molecular Weight Heparin (LMWH).”

University of Iowa Hospitals & Clinics: “Possible warfarin side effects and signs of a blood clot.”

Saint Luke’s: “Using Blood Thinners.” 

National Institutes of Health: “Vitamin K.”

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