What are Low-Molecular-Weight Heparins?

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  • Written By: B. Chisholm
  • Edited By: Nancy Fann-Im
  • Last Modified Date: 04 October 2019
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Low-molecular-weight heparins (LMWH) are a class of drugs used for treatment of patients with thrombosis, or blood clots, and as prophylaxis for those who are at risk of thrombosis. They are injectable drugs, usually given sub-cutaneously. Enoxaparin, dalteparin and nadroparin are all examples of low-molecular-weight heparins and go by different trade names in different countries. They are available by prescription only.

Traditionally anti-coagulation was achieved using standard heparin, which necessitated hospitalization and close monitoring of the blood parameters. With low-molecular-weight heparins this is no longer always necessary. Derived from standard heparin, low-molecular-weight heparins have a far lower molecular weight than standard heparin. This gives them distinctly different properties, including a predictable dose response that does not always need monitoring.

Thrombosis and most often deep vein thromboses (DVT) are common in general practice. Initial treatment involves the use of low-molecular-weight heparins, usually in combination with an oral anticoagulant such as warfarin. The low-molecular-weight heparin treats the clot and thins the blood while the correct warfarin dosage is established.

Low-molecular-weight heparins are also used prophylactically in patients with a high risk of clotting. This includes those undergoing hip or knee replacement and bed-ridden patients. They are also used after certain types of heart attacks and during some cardiac surgeries.


The way in which low-molecular-weight heparins work is by their action on anti-Xa factor. Blood clotting involves a complex cascade of enzymes and actions. They increase the antithrombin III-mediated inhibition of the formation and activity of factor Xa, a key player in clot formation. In some cases, anti-Xa will be monitored, but this is not routine.

The drugs are administered sub-cutaneously, either once or twice a day, and can, with some instruction and training by a medical professional, be self-administered. The dose depends on which drug in the class is prescribed and will differ accordingly. Any concomitant medication, diseases, pregnancy or lactation should be discussed with a medical professional before use, as interactions may occur.

Low-molecular-weight heparins may increase the risk of bleeding and make the patient more susceptible to bruising. Any signs of bleeding, including bruising and red or black tarry stool or urine should be reported to the doctor immediately. Low-molecular-weight heparins are usually used short-term. The duration of therapy will be determined by the prescribing doctor.


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