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[Pharmacology](https://durgical.com/en/pharmacology/) · Class

# Heparins

Injectable anticoagulants that boost antithrombin, the body's own clot brake. By vein they act at once.

Updated 2026-11-01

## What they are

Injectable anticoagulants made of sugar chains: unfractionated heparin (long, mixed chains) and low-molecular-weight heparins (shorter chains).

## Mechanism of action

Heparin binds antithrombin through a five-sugar sequence (a pentasaccharide) and greatly speeds up its inhibition of clotting factors, mainly thrombin (IIa) and factor Xa. Unfractionated heparin acts on both about equally; low-molecular-weight heparins such as enoxaparin act much more on Xa than on IIa.

Target: Antithrombin (and through it, thrombin and factor Xa)

## Key points

- Unfractionated heparin blocks IIa and Xa about equally; enoxaparin has a far higher anti-Xa to anti-IIa ratio.
- Unfractionated heparin is followed with the aPTT; for enoxaparin the PT and aPTT are not adequate, and anti-Xa levels are used when needed.
- Heparin-induced thrombocytopenia (HIT): antibodies to heparin-platelet factor 4 complexes, falling platelets and possible thrombosis, usually about 5 to 9 days in.
- Protamine neutralises unfractionated heparin but only partly reverses the anti-Xa activity of enoxaparin.
- Enoxaparin leaves partly through the kidneys, so its effect builds up in severe kidney impairment.

## Drugs in this class

- [**Heparin** An injectable anticoagulant that boosts antithrombin, the body's own clot brake. By vein it acts at once and is… Antithrombin](https://durgical.com/en/pharmacology/heparin/)
- [**Enoxaparin** A low-molecular-weight heparin injected under the skin. It acts mainly on factor Xa and needs no routine aPTT checks. Antithrombin (and through it, mainly factor Xa)](https://durgical.com/en/pharmacology/enoxaparin/)
- [**Fondaparinux** A synthetic five-sugar molecule, injected under the skin, that makes antithrombin block factor Xa. It has no effect on… Antithrombin (and through it, factor Xa only)](https://durgical.com/en/pharmacology/fondaparinux/)
- [**Dalteparin** A low-molecular-weight heparin injected under the skin to prevent and treat venous clots, including clots linked to… Antithrombin (and through it, mainly factor Xa)](https://durgical.com/en/pharmacology/dalteparin/)

## Others in the class

- enoxaparin
- dalteparin
- nadroparin
- tinzaparin
- bemiparin

## Test yourself

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

## How does the activity of enoxaparin differ from that of unfractionated heparin?

Enoxaparin acts much more on factor Xa than on thrombin (a far higher anti-Xa to anti-IIa ratio), while unfractionated heparin acts on both about equally.

## Which laboratory test follows unfractionated heparin, and which follows warfarin?

Unfractionated heparin: the aPTT. Warfarin: the prothrombin time reported as the INR.

## What causes heparin-induced thrombocytopenia?

Antibodies against complexes of heparin and platelet factor 4. Platelets fall and clots can form (HITT); it usually starts about 5 to 9 days after heparin begins.

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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](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=cb1c1e7a-c9ca-4a07-8833-e45ce436d287) (DailyMed (U.S. National Library of Medicine), 2026); [DailyMed - ENOXAPARIN SODIUM injection](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=b4728749-8453-4f07-9fbc-a1f3e9d2708b) (DailyMed (U.S. National Library of Medicine), 2026); [Heparin Injection: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a682826.html) (MedlinePlus (U.S. National Library of Medicine), 2025); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=B01AB&showdescription=no) (WHO Collaborating Centre for Drug Statistics Methodology, 2026).

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Representative model, for educational purposes only.

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