Heparin exerts its anticoagulant effect by binding to and enhancing the activity of antithrombin III (ATIII), a natural inhibitor of clotting factors. The mechanism of action involves:

1. Inhibition of Thrombin and Factor Xa:
   • Heparin binds to ATIII, causing a conformational change that enhances its ability to inactivate thrombin (factor IIa) and factor Xa.
   • By inhibiting these clotting factors, heparin prevents the formation of blood clots.
2. Rapid Onset of Action:
   • Heparin has an immediate anticoagulant effect when administered intravenously (IV) or subcutaneously (SC).
3. Short Half-Life:
   • The anticoagulant effect of heparin is short-lived, with a half-life of approximately 1–2 hours for IV administration and 3–4 hours for SC administration.

Heparin is approved for the following conditions:

1. Prevention and Treatment of Venous Thromboembolism (VTE):
   • Used to prevent and treat deep vein thrombosis (DVT) and pulmonary embolism (PE).
2. Acute Coronary Syndromes (ACS):
   • Used in the management of unstable angina and non-ST-segment elevation myocardial infarction (NSTEMI) to prevent further thrombotic events.
3. Prevention of Clotting in Extracorporeal Circuits:
   • Used in patients undergoing hemodialysis, cardiopulmonary bypass, or extracorporeal membrane oxygenation (ECMO) to prevent clotting in the extracorporeal circuit.
4. Disseminated Intravascular Coagulation (DIC):
   • Used in some cases of DIC to prevent further clot formation and consumption of clotting factors.

The dosage of heparin varies depending on the indication, route of administration, and patient-specific factors. Below are general guidelines:

1. Prevention of VTE:
   • Subcutaneous (SC) Administration:
     • Standard Dose: 5,000 units SC every 8–12 hours.
     • Duration: Typically continued until the patient is ambulatory or the risk of VTE is reduced.
2. Treatment of VTE:
   • Intravenous (IV) Administration:
     • Initial Bolus: 80 units/kg IV, followed by a continuous infusion of 18 units/kg/hour.
     • Monitoring: Adjust the infusion rate based on the activated partial thromboplastin time (aPTT), typically targeting an aPTT ratio of 1.5–2.5 times the control value.
3. Acute Coronary Syndromes (ACS):
   • IV Administration:
     • Initial Bolus: 60–70 units/kg IV (maximum 5,000 units), followed by a continuous infusion of 12–15 units/kg/hour.
     • Monitoring: Adjust the infusion rate based on the aPTT.
4. Special Populations:
   • Renal Impairment: Use with caution in patients with severe renal impairment. LMWH may be preferred in these patients.

Elderly: No dose adjustment is required unless renal function is impaired

Heparin is generally well-tolerated, but some side effects may occur. These include:

1. Common Side Effects:
   • Bleeding (e.g., hematoma at the injection site, nosebleeds)
   • Injection site reactions (e.g., pain, redness, swelling)
   • Thrombocytopenia (low platelet count)
2. Less Common Side Effects:
   • Hypersensitivity reactions (e.g., rash, itching)
   • Elevated liver enzymes
3. Rare but Serious Side Effects:
   • Major Bleeding: Gastrointestinal bleeding, intracranial hemorrhage.
   • Heparin-Induced Thrombocytopenia (HIT): A severe immune-mediated reaction that can lead to thrombosis.
   • Osteoporosis: With long-term use, especially in high doses.

1. Contraindications:
   • Hypersensitivity to heparin or pork products.
   • Active clinically significant bleeding.
   • History of heparin-induced thrombocytopenia (HIT).
2. Warnings:
   • Bleeding Risk: Heparin increases the risk of bleeding. Use with caution in patients with a history of bleeding disorders or conditions that predispose to bleeding.
   • HIT: Monitor platelet counts regularly to detect HIT, especially in patients receiving heparin for more than 5 days.
   • Pregnancy and Lactation: Heparin is generally considered safe during pregnancy and breastfeeding, as it does not cross the placenta or enter breast milk.
3. Monitoring:
   • Regular monitoring of aPTT for patients receiving IV heparin.
   • Regular platelet counts to detect HIT.
   • Watch for signs of bleeding (e.g., unusual bruising, blood in stool or urine).

Heparin interacts with several medications, including:

1. Other Anticoagulants (e.g., warfarin, dabigatran): Increased risk of bleeding.
2. Antiplatelet Agents (e.g., aspirin, clopidogrel): Increased risk of bleeding.
3. NSAIDs (e.g., ibuprofen): Increased risk of gastrointestinal bleeding.
4. Thrombolytics (e.g., alteplase): Increased risk of bleeding.

Heparin is available under its generic name as well as brand names. Some common formulations include:

• Generic Name: Heparin sodium
• Brand Names: Heparin (commonly used worldwide), and others depending on the region.