Haematology
Heparins
Low-molecular-weight and unfractionated heparin — Injectable blood-thinners used to prevent and treat clots, including in hospital and pregnancy.
Education and reference only. This is a plain-language class overview — it deliberately contains no doses. Always check the current Summary of Product Characteristics (SmPC), the BNF and your local formulary before prescribing or administering any medicine.
What it is
Heparins are injectable anticoagulants used to prevent venous thromboembolism in hospital, to treat established clots, and in specific settings such as pregnancy and around procedures.
How it works
They boost the body's natural clotting inhibitor (antithrombin), which switches off key clotting factors. LMWH gives a predictable effect from a weight-based dose; unfractionated heparin acts fast and wears off quickly, which is useful when tight control is needed.
In practice
In practice low-molecular-weight heparin (LMWH) has become the everyday workhorse — predictable, given by injection once or twice daily, and widely used to prevent clots in hospital inpatients and to treat established DVT/PE, including in pregnancy where it is the anticoagulant of choice. Renal function matters because LMWH is cleared by the kidneys (the dose is reduced, or unfractionated heparin used, in severe impairment), and platelet awareness matters because of the rare immune reaction HIT. Unfractionated heparin, given by infusion, is reserved for situations needing rapid on/off control.
Examples
Practical use
How to take it & use it well
- Low-molecular-weight heparins such as enoxaparin and dalteparin are given by injection just under the skin, often into the tummy; if injecting at home, follow the technique you were shown.
- Rotate injection sites and do not rub the area afterwards, as some bruising at the site is normal.
- Use the pre-filled syringe as supplied and do not expel the small air bubble in treatment syringes unless told to, as it helps deliver the full dose.
- Take it at around the same time each day and keep to your prescribed schedule, as these medicines prevent and treat dangerous blood clots.
- Tell any clinician you are on a heparin before procedures, dental work or new medicines, and report unusual bleeding or bruising.
- Do not stop without advice; your clinician may switch you to or from tablet anticoagulants and will plan the timing carefully.
Common uses
- Prevention of clots in hospital inpatients
- Treatment of DVT and pulmonary embolism
- Anticoagulation in pregnancy
- Bridging and procedural anticoagulation
Monitoring
- Signs of bleeding; renal function
- Platelet count where HIT risk applies
- Anti-Xa levels only in selected cases (e.g. extremes of weight, renal impairment, pregnancy)
Weighing it up
Advantages & disadvantages
Advantages
- They reliably prevent and treat blood clots such as deep vein thrombosis and clots on the lungs.
- Once-daily or twice-daily injections often need less monitoring than older intravenous heparin.
- They can be used in pregnancy when an anticoagulant is needed, as they do not cross the placenta.
- They are widely used to prevent clots after surgery and during hospital stays.
Disadvantages
- They require injections, which some people find uncomfortable or difficult to self-administer.
- Bleeding and bruising are the main risks, and serious bleeding can occur.
- A rare immune reaction called heparin-induced thrombocytopenia (HIT) can lower platelets and paradoxically cause clots.
- Doses often need reducing or avoiding when kidney function is poor, as the drug can build up.
Key safety principles
What to watch for
- Bleeding is the main risk; balance against the thrombotic risk being treated.
- Heparin-induced thrombocytopenia (HIT) — a rare immune reaction; monitor platelets where indicated.
- LMWH is renally cleared — reduce dose or use unfractionated heparin in severe renal impairment.
Key interactions
What to avoid or check alongside
- Other anticoagulants and antiplatelets such as aspirin, clopidogrel or warfarin add to the bleeding risk.
- Anti-inflammatory painkillers (NSAIDs) increase the chance of bleeding, including from the stomach.
- SSRI and SNRI antidepressants can modestly raise bleeding risk when combined.
- Other medicines that affect potassium may add to the small risk of raised potassium with heparins.
- Herbal products such as high-dose fish oil, ginkgo or garlic supplements may further increase bleeding tendency.
Patient & carer advice
- You or a nurse will be shown how to give the injection and rotate sites
- Report unusual bruising, bleeding or black stools
- Bruising at the injection site is common and harmless
Use with
Related clinical calculators
Dose and risk decisions for this class often depend on renal function, weight or bleeding/stroke risk. These tools help:
Answers
Heparins: frequently asked questions
Why is heparin given as an injection rather than a tablet?
Heparins are not absorbed well from the gut, so they are given by injection under the skin. Low-molecular-weight heparins are designed for convenient once or twice daily injections that many people can give themselves at home.
What is heparin-induced thrombocytopenia (HIT)?
HIT is a rare immune reaction to heparin that lowers the platelet count and, paradoxically, can trigger dangerous clots. It usually appears after several days of treatment. Your clinician monitors for it and will switch treatment if it is suspected.
Can I use heparin in pregnancy?
Low-molecular-weight heparins are commonly used in pregnancy when an anticoagulant is needed because they do not cross the placenta to the baby. Treatment must be supervised by your maternity and clinical team, who will tailor it to you.
Is bruising at the injection site normal?
Yes, small bruises where you inject are common and usually harmless. Rotating sites and not rubbing the area can help. Report larger bruises, bleeding that will not stop, blood in urine or stool, or any severe or unusual bleeding.
Why does kidney function matter with heparins?
Some heparins are cleared by the kidneys, so in people with reduced kidney function the medicine can build up and increase bleeding risk. Your clinician may adjust the dose, monitor more closely, or choose a different option.
Authoritative sources
Always verify against the source
This overview is for orientation. For doses, interactions, contra-indications and the full monograph, use:
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