Heparin !

An injectable anticoagulant that boosts antithrombin, the body's own clot brake. By vein it acts at once and is followed with the aPTT.

Updated 2026-11-01

Class: Heparins
Target: Antithrombin
ATC code: B01AB01
WHO Model List of Essential Medicines

Also called: unfractionated heparin, heparin sodium

AntithrombinHeparin

The figure shows antithrombin bound to the heparin pentasaccharide, the five-sugar sequence that binds and activates it (not a full heparin chain).

Target in 3D: Antithrombin. PDB structure 1AZX (RCSB PDB, public domain). View the structure at RCSB PDB

Mechanism of action

Heparin binds antithrombin and greatly speeds up its inhibition of clotting factors, mainly thrombin (IIa) and factor Xa. The binding happens through a specific five-sugar sequence (the pentasaccharide in the figure). Heparin does not dissolve existing clots, but it stops them from growing.

The heparin pentasaccharide binds antithrombin
Antithrombin then works far faster
Thrombin (IIa) and factor Xa are inactivated
Less fibrin: the aPTT lengthens and clots stop growing

Pharmacokinetics

Absorption: Not taken by mouth; given into a vein or under the skin, not into muscle.
Metabolism: Not broken down by enzymes in the usual way; taken up by the liver and the reticuloendothelial system.
Elimination: Mainly cleared by that cellular uptake.
Half-life: Short and dose-dependent: about half an hour to two hours.
Onset and duration: Immediate by vein.

Indications

Prevention and treatment of deep vein thrombosis. Deep vein thrombosis →
Treatment of pulmonary embolism, as part of venous thromboembolism care. Pulmonary embolism →
Acute ischaemic heart disease, including unstable angina and myocardial infarction. Myocardial Infarction (Heart Attack) →
Keeping catheters clot-free; selected situations in pregnancy.

Adverse effects

Common

Bleeding
Redness, pain or bruising at the injection site
Hair loss

Contraindications

Current or past heparin-induced thrombocytopenia
Uncontrolled active bleeding
Allergy to heparin or pork products

Key interactions

Aspirin, clopidogrel and other antiplatelet drugs: Add to bleeding risk.
NSAIDs: Add to bleeding risk.
Warfarin: Heparin can affect the prothrombin time, so INR samples are timed away from heparin given by vein.

What is monitored

aPTT, which guides the regimen
Platelet count before and during treatment, looking for HIT
Signs of bleeding

Test yourself

Think of the answer, then tap the question to see it.

How is unfractionated heparin reversed?

With protamine sulfate, given slowly into a vein, which neutralises heparin. It only partly reverses the anti-Xa activity of low-molecular-weight heparins.

A patient on heparin for 7 days has a sharp fall in platelets and a new leg clot. What is suspected?

Heparin-induced thrombocytopenia with thrombosis: all heparin is stopped and a non-heparin anticoagulant is used.

Does heparin dissolve a clot that is already there?

No. It stops the clot from growing and new clots from forming; the body's own fibrinolysis then breaks the clot down.

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: DailyMed - HEPARIN SODIUM- heparin sodium injection, solution (DailyMed (U.S. National Library of Medicine), 2026); Heparin Injection: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2025); Partial Thromboplastin Time (PTT) Test: MedlinePlus Medical Test (MedlinePlus (U.S. National Library of Medicine), 2026); eEML - Electronic Essential Medicines List (World Health Organization, 2025).

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Representative model, for educational purposes only.