Low-molecular-weight heparins for deep vein thrombosis

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Low-molecular-weight heparins for deep vein thrombosis


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A type of medicine that is similar to low-molecular-weight heparin is fondaparinux (Arixtra), which is a type of antithrombotic medicine called a selective Factor Xa inhibitor.

How It Works

Normally, when an injury that causes bleeding occurs, the body sends out signals that cause blood to clot at the wound, and the clot naturally breaks down as the wound heals. A person prone to abnormal clotting has an imbalance between clot formation and clot breakdown.

Anticoagulant medicines prevent new clots from forming and prevent existing clots from growing (extending) by stopping the production of certain proteins that are needed for blood to clot. They do not break up or dissolve existing blood clots.

Why It Is Used

Low-molecular-weight heparins can be used to treat a deep vein thrombosis. When used to either prevent or treat a blood clot, they are given by injection just under the skin once or twice each day. Unlike with other forms of anticoagulants, periodic blood tests are usually not needed to monitor how well the medications are working.

How Well It Works

Low-molecular-weight heparin can be used to treat or prevent a deep vein thrombosis. When used for treatment, low-molecular-weight heparins prevent new blood clots from forming and prevent existing clots from getting larger.

  • This allows the normal body systems to dissolve the clots that are already formed.
  • This also reduces the risk of pulmonary embolism.

Side Effects

Bleeding is the most common side effect of low-molecular-weight heparins.

Know the signs of bleeding

Call 911 if:

  • You cough up blood.
  • You vomit blood or what looks like coffee grounds.
  • You pass maroon or very bloody stools.

Call your doctor right away if:

  • You have new bruises or blood spots under your skin.
  • You have a nosebleed that doesn't stop quickly.
  • Your gums bleed when you brush your teeth.
  • You have blood in your urine.
  • Your stools are black and look like tar or have streaks of blood.
  • You have heavy period bleeding or vaginal bleeding when you are not having your period.

If you are injured, apply pressure to stop the bleeding. Realize that it will take longer than you are used to for the bleeding to stop. If you can't get the bleeding to stop, call your doctor.

Heparin shots may cause irritation, pain, or bruising at the injection site.

See Drug Reference for a full list of side effects. (Drug Reference is not available in all systems.)

What To Think About

Low-molecular-weight heparins (LMWH) are often used in higher doses to treat deep vein thrombosis rather than prevent it. In most cases, LMWH is preferred over unfractionated heparin (UH), because it is effective and can be given at home. Most people can be treated with LMWH while at home, because:

  • It is given as an injection only 1 or 2 times a day.
  • It does not usually require blood tests to monitor its effects.

Unfractionated heparin usually requires a hospital stay, because it is given as a continuous infusion, or IV, and frequent monitoring is often needed.

When heparins are given by injection under the skin, localized irritation, pain, or bruising can occur.

You will need to take extra steps to avoid bleeding problems while taking heparin. You will need to:

Low-molecular-weight heparin can be used in pregnant women.

Your doctor may not have you take LMWH if you have any of the following:

  • Increased bleeding risk, such as from active ulcers
  • Kidney or liver disease
  • Very low or high body weight (more study is needed on the proper dosage and how it should be monitored)
  • Problems with administering the drug, such as injection technique, or trouble getting the medicine and related supplies

Complete the new medication information form (PDF) (What is a PDF document?) to help you understand this medication.


By Healthwise Staff
Primary Medical Reviewer E. Gregory Thompson, MD - Internal Medicine
Specialist Medical Reviewer Jeffrey S. Ginsberg, MD, MD - Hematology
Last Revised February 26, 2010

This information does not replace the advice of a doctor. Healthwise, Incorporated disclaims any warranty or liability for your use of this information.