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Blood Thinners
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Blood Heart Circulation

Blood Thinners

Blood thinners are medications used to prevent blood clots from forming or growing, which helps reduce the risk of heart attacks and strokes.

What are blood thinners?

Blood thinners are medicines that prevent blood clots from forming. They do not break up clots that you already have, but they can stop those clots from getting bigger. It's important to treat blood clots, because clots in your blood vessels and heart can cause heart attacks, strokes, and blockages.

Who needs blood thinners?

You may need a blood thinner if you have:

  • Certain heart or blood vessel diseases
  • An abnormal heart rhythm called atrial fibrillation
  • A heart valve replacement
  • A risk of blood clots after surgery
  • Congenital heart defects

Types of blood thinners

There are different types of blood thinners:

  • Anticoagulants, such as heparin or warfarin (also called Coumadin), slow down your body's process of making clots.
  • Antiplatelets, such as aspirin and clopidogrel, prevent blood cells called platelets from clumping together to form a clot. Antiplatelets are mainly taken by people who have had a heart attack or stroke.

Safe Usage

When you take a blood thinner, follow the directions carefully. Blood thinners may interact with certain foods, medicines, vitamins, and alcohol. Make sure that your health care provider knows all of the medicines and supplements you are using.

You may need regular blood tests to check how well your blood is clotting. It is important to make sure that you're taking enough medicine to prevent clots, but not so much that it causes bleeding.

Side Effects

Bleeding is the most common side effect of blood thinners. They can also cause an upset stomach, nausea, and diarrhea. Other possible side effects can depend on which type of blood thinner that you are taking.

Call your provider if you have any sign of serious bleeding, such as:

  • Menstrual bleeding that is much heavier than normal
  • Red or brown urine
  • Bowel movements that are red or black
  • Bleeding from the gums or nose that does not stop quickly
  • Vomit that is brown or bright red
  • Coughing up something red
  • Severe pain, such as a headache or stomachache
  • Unusual bruising
  • A cut that does not stop bleeding
  • A serious fall or bump on the head
  • Dizziness or weakness

Frequently Asked Questions

If you're taking certain blood thinners, especially warfarin, your doctor will likely require regular blood tests, such as an INR (International Normalized Ratio) test. This test measures how long it takes for your blood to clot and helps your doctor adjust your dosage to ensure it's effective enough to prevent clots but not so high that it causes excessive bleeding. Newer blood thinners may require less frequent monitoring.
When taking blood thinners, watch for signs like unusually heavy or prolonged bleeding from cuts, frequent nosebleeds, bleeding gums, heavy menstrual periods, or easy bruising. More serious signs requiring immediate medical attention include blood in your urine (pink or red), blood in your stool (red or black and tarry), coughing up blood, vomiting blood, severe headaches, sudden weakness or numbness, or any bleeding that won't stop.
It's crucial to discuss all medications, supplements, and significant dietary changes with your doctor. Certain foods rich in Vitamin K (like leafy green vegetables) can interact with some anticoagulants like warfarin. Many over-the-counter pain relievers (like ibuprofen or naproxen), herbal supplements (like ginkgo biloba or St. John's wort), and even alcohol can increase your bleeding risk. Always check with your healthcare provider or pharmacist before starting anything new.
Blood thinners work by either slowing down your body's ability to form new clots (anticoagulants) or by preventing blood cells called platelets from clumping together (antiplatelets). While they don't dissolve existing clots, they stop those clots from growing larger and prevent new, dangerous clots from forming in your blood vessels or heart. This significantly reduces the risk of clots traveling to the brain (causing a stroke) or blocking blood flow to the heart (causing a heart attack).
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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