In the treatment of lower extremity deep vein thrombosis, vascular surgeons often prescribe a drug called low molecular heparin to patients on an outpatient basis, allowing them to inject it on an outpatient basis without having to either be hospitalized or have their heparin dosage monitored. Why is the use of low molecular heparin so convenient? Ordinary heparin is a heterogeneous body of different sizes with molecular weights ranging from 5,000 to 3,000. The pharmacological effects of heparin fragments of different molecular weights are different, so the mechanism by which ordinary heparin exerts its anticoagulant effect is very complicated. In contrast, low-molecular heparin is a heparin fragment with a molecular weight of 4,000 to 6,500 that is cleaved and purified from common heparin. Its mechanism of action is relatively simple, and it mainly inhibits the activity of coagulation factor X, while the inhibition of thrombin is weak, so there are few bleeding complications and the dosage can not be monitored. In addition it has a longer half-life than regular heparin and can be administered by subcutaneous injection.