Heparin (unfractionated)
Heparin sodium
Also known as: Hepsal · UFH · Unfractionated heparin · Heparin sodium · Heparinum
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Dosing by species
Dosing data for this drug is not yet available in the VetSheet formulary. Every other section below is complete. Consult the current product label before prescribing.
Overview
Heparin is an unfractionated injectable anticoagulant used in veterinary medicine to prevent and treat thromboembolic disease and to maintain the patency of intravenous catheters. It relies on the presence of endogenous antithrombin III (AT III) to exert its effects.
Clinical Warning: Heparin carries a significant risk of hemorrhage. It does not lyse existing clots but prevents their extension.
Clinical Pearl: Because heparin requires AT III to function, it may be ineffective in patients with severe protein-losing nephropathy or enteropathy where AT III is depleted.
Mechanism of action
Heparin exerts its effects primarily by enhancing the binding of antithrombin III (AT III) to factors IIa, IXa, Xa, XIa and XlIa.
- AT III/clotting factor complex formation → subsequent removal by the liver.
- Inactivates thrombin → blocks the conversion of fibrinogen to fibrin.
- Inhibition of Factor XII activation → prevents the formation of stable fibrin clots.
Note: Heparin is only effective if adequate AT III is present. It does not significantly change the concentrations of clotting factors, nor does it lyse pre-existing clots.
Safety & warnings
Contraindications
- Active bleeding or severe bleeding disorders
- Severe thrombocytopenia
- Recent neurosurgery or ocular surgery
- Known hypersensitivity to heparin
Adverse effects
- Hemorrhage (most common)
- Heparin-induced thrombocytopenia (HIT)
- Injection site reactions (hematoma, pain)
- Hypersensitivity reactions
Precautions
Important: Do NOT administer intramuscularly (IM) due to the high risk of severe hematoma formation. Monitor activated partial thromboplastin time (aPTT) closely during therapeutic use. Extreme caution is advised in patients with severe hepatic or renal disease.
Drug interactions
Increased risk of bleeding due to platelet inhibition
Increased risk of gastrointestinal ulceration and bleeding
Synergistic anticoagulant effect; increased bleeding risk
Increased risk of bleeding
Monitoring
- aPTT (Activated Partial Thromboplastin Time) - typically target 1.5 to 2.5 times baseline
- Platelet count
- Packed Cell Volume (PCV) and Total Protein (TP)
- Clinical signs of bleeding (mucous membranes, feces, urine)
Pharmacokinetics
Half-life
Absorption
Not absorbed orally. Well absorbed following subcutaneous (SC) or intravenous (IV) administration.
Distribution
Extensively bound to plasma proteins. Does not cross the placenta or enter maternal milk.
Metabolism
Metabolized by the liver and the reticuloendothelial system.
Elimination
Excreted primarily in the urine. Clearance is dose-dependent.
Overdose
The primary sign of overdosage is hemorrhage (e.g., epistaxis, hematuria, melena, petechiae).
- Mild to moderate overdose: Discontinue heparin therapy; due to its short half-life, coagulation parameters often normalize quickly.
- Severe, life-threatening hemorrhage: Administer protamine sulfate as a specific reversal agent (antidote). Administer slowly IV to avoid hypotension and anaphylactoid reactions.
Available products
Formulations
- Injectable solution: 1,000 IU/ml
- Injectable solution: 5,000 IU/ml
- Injectable solution: 10,000 IU/ml
- Injectable solution: 25,000 IU/ml
- Saline flush solution: 10 IU/ml
- Saline flush solution: 100 IU/ml
Veterinary
- Heparin flush solutions (10 IU/ml, 100 IU/ml)
Human-labeled
- Heparin sodium injection (1,000 to 25,000 IU/ml)
- Hepsal
Regulatory status
Human-approved products are commonly used off-label in veterinary medicine.
Available as POM (Prescription Only Medicine).
POM-V or POM-VPS depending on the specific formulation.
No regulatory data for: 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Store at room temperature (15-30°C). Do not freeze. Inspect for particulate matter or discoloration prior to administration.
Client information
Heparin is used to prevent dangerous blood clots.
- Watch for bleeding: The most common side effect is bleeding. Contact your veterinarian immediately if you notice pale gums, unexplained bruising, nosebleeds, blood in the urine, or dark/tarry stools.
- Prevent injury: Keep your pet calm and avoid rough play or activities that could lead to injury or bruising while they are on this medication.
- Do not mix medications: Never give your pet aspirin, NSAIDs, or any other over-the-counter medications without consulting your veterinarian, as this can cause fatal bleeding.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.