An injectable anticoagulant used in hospital, reversed with protamine
Heparin (unfractionated)
An injectable anticoagulant ('blood thinner') used in hospital to prevent and treat blood clots.
What is Heparin (unfractionated)?
Heparin (the unfractionated form) is an injectable anticoagulant, or 'blood thinner', used mainly in hospital to prevent and treat blood clots. It is given into a vein as a continuous infusion or under the skin, and works quickly. Its main concern is bleeding. A second important risk is a reaction called heparin-induced thrombocytopenia (HIT), where the platelet count drops and, paradoxically, clotting risk rises, so platelets are monitored. When given as a continuous infusion it needs blood tests (the APTT) to guide the dose, and its effect can be reversed with a medicine called protamine.
Education and reference only. This is a plain-language guide to Heparin (unfractionated) — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Heparin is an anticoagulant, a medicine that slows down the blood's clotting. The 'unfractionated' form discussed here is given by injection, either as a continuous drip into a vein or as an injection under the skin, and is used mainly in hospital. It acts fast, which makes it useful in urgent situations and in settings where its effect may need to be turned on, adjusted or reversed quickly. It is used to prevent and treat blood clots in the veins and lungs and in various hospital procedures. It is given and monitored by healthcare staff.
How it works
Heparin boosts the activity of a natural substance in the blood called antithrombin, which switches off some of the key proteins (clotting factors) that the body uses to form clots. By dampening these clotting factors, heparin makes the blood less likely to clot, helping to prevent new clots forming and stopping existing ones from growing. Because it acts quickly and wears off relatively quickly, its effect can be adjusted by changing the infusion rate, and it can be reversed with protamine if needed. When infused, blood tests guide how much is given.
Company & origin
Originated / developed by: Generic.
An injectable blood-thinning medicine used mainly in UK hospitals, given into a vein or under the skin and closely monitored.
What it treats
Conditions Heparin (unfractionated) is used for
Practical use
How to take Heparin (unfractionated)
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- It is given by healthcare staff, either as a continuous drip into a vein or as an injection under the skin.
- Attend or allow the blood tests needed, such as the APTT for an infusion and platelet counts, to guide treatment safely.
- Tell staff straight away about any unusual bruising, bleeding, blood in urine or stool, or nosebleeds.
- Mention all your other medicines, especially anything that affects bleeding such as aspirin or other blood thinners.
- Report any new or worsening pain, swelling or breathing problems, which need urgent assessment.
Weighing it up
Advantages & disadvantages of Heparin (unfractionated)
Advantages
- A fast-acting, effective anticoagulant for preventing and treating blood clots.
- Its effect can be adjusted quickly by changing the infusion rate, and reversed with protamine if needed.
- Useful in urgent situations and where rapid control of clotting is important.
Disadvantages
- The main risk is bleeding, which can be serious.
- Can cause heparin-induced thrombocytopenia (HIT), needing platelet monitoring.
- When infused it needs regular blood tests (the APTT) and is given mainly in hospital.
Practical use
Good to know
The dominant concern with heparin is bleeding, since it is a powerful blood thinner; signs such as unusual bruising, blood in the urine or stool, nosebleeds that will not stop, or any heavy or unexplained bleeding need urgent attention. A second, distinctive risk is heparin-induced thrombocytopenia (HIT), an immune reaction in which the platelet count falls and, despite the low platelets, the blood can actually become more likely to clot; for this reason platelet counts are monitored and heparin is stopped if HIT is suspected. When heparin is given as a continuous infusion into a vein, a blood test called the APTT is used to check the level of thinning and guide the dose. A useful safety feature is that its effect can be reversed with a medicine called protamine in an emergency. Heparin is almost always managed by hospital staff.
Who should not take it / use with caution
- People with active serious bleeding or a very high bleeding risk should not be given it.
- People who have had a serious allergic reaction to heparin, or a past episode of HIT, should not receive it.
- It is used with great caution in people with certain bleeding disorders, recent surgery or severe high blood pressure.
- It is given and supervised by healthcare professionals, with monitoring throughout.
Monitoring
- Blood tests (the APTT) to guide the dose when heparin is given as an infusion.
- Regular platelet counts to watch for heparin-induced thrombocytopenia (HIT).
- Watching for signs of bleeding and checking kidney function and potassium where appropriate.
Side effects
- Bleeding or unusual bruising, which can range from minor to serious.
- A fall in the platelet count, including the immune reaction HIT.
- Bruising, pain or irritation where injections are given under the skin.
- With long-term use, thinning of the bones, and rarely raised potassium or liver test changes.
Key interactions
- Other blood thinners and antiplatelets such as aspirin add to the risk of bleeding.
- NSAID painkillers and some other medicines can also increase bleeding risk.
- Tell staff about all your medicines, including over-the-counter and herbal products.
Available as: A solution given into a vein as an infusion or by injection under the skin.
Answers
Heparin (unfractionated): frequently asked questions
What is heparin used for?
It is an injectable anticoagulant used mainly in hospital to prevent and treat blood clots in the veins and lungs and during certain procedures.
What is the main risk?
Bleeding is the main risk, so any unusual bruising or bleeding should be reported to staff straight away.
What is HIT?
Heparin-induced thrombocytopenia (HIT) is a reaction where the platelet count drops and clotting risk can rise; platelets are monitored and heparin is stopped if it is suspected.
Why do I need blood tests?
When heparin is given as a continuous infusion, a test called the APTT checks how thin the blood is and guides the dose, alongside platelet counts.
Can its effect be reversed?
Yes, in an emergency a medicine called protamine can be used to reverse heparin's blood-thinning effect.
The wider class
About Anticoagulant (unfractionated heparin)
Heparin (unfractionated) belongs to the anticoagulant (unfractionated heparin) class. For how the class as a whole works, its shared safety principles and monitoring, see the full guide.
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Authoritative sources
- BNF
- NICE CKS
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