Anticoagulant Drugs

Overview – Anticoagulant Drugs

Anticoagulant drugs are a critical part of modern medical therapy, used extensively to prevent and treat thromboembolic conditions. These agents work by interrupting different stages of the coagulation cascade, leading to reduced clot formation. This page focuses on the mechanisms, clinical use, pharmacokinetics, and adverse effects of key anticoagulants including heparin, low molecular-weight heparins, and warfarin, highlighting their differences for safe and effective use.

Heparin

Mechanism of Action

  • Binds and activates Antithrombin III (AT-III)
  • Heparin–AT-III complex inactivates:
    • Thrombin (Factor IIa) → ↓ activation of fibrinogen
    • Factor Xa → ↓ conversion of prothrombin to thrombin
    • Factors IX, XI, XII → Intrinsic pathway inhibition

Indications

Side Effects

  • Bleeding
    • Antidote = Protamine sulfate
  • Heparin-Induced Thrombocytopenia (HIT)
    • Type I: non-immune, transient
    • Type II: immune-mediated, thrombotic risk
  • Others:
    • Osteoporosis
    • Allergic reactions (e.g. skin necrosis)
    • Hyperkalaemia (via hypoaldosteronism)

Pharmacokinetics / Notes

  • Rapid onset (IV)
  • Not orally bioavailable
  • Requires hospital administration and monitoring (aPTT)
Heparin
SteveKong3, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

Low Molecular-Weight Heparins (LMWHs)

What Are They?

  • Smaller fragments of heparin (e.g. enoxaparin)
  • Bind to AT-III → selective inhibition of Factor Xa only
  • Cannot inactivate thrombin due to small size

Advantages over Unfractionated Heparin

  • Longer half-life
  • Self-administered via subcutaneous injection
  • No routine monitoring required
  • Predictable dose-response profile
  • Suitable for outpatient use
  • More expensive than standard heparin

Adverse Effects

  • Lower risk of thrombocytopenia than unfractionated heparin
  • Bleeding (less frequent but still possible)
anticoagulant drugs: Low Molecular-Weight Heparins
Image credit: Rang, Dale, et al. Pharmacology; available from: https://amzn.to/3Hr51dO

Warfarin

Mechanism of Action

  • Vitamin K antagonist
  • Competitively inhibits vitamin K epoxide reductase
  • ↓ Synthesis of vitamin K-dependent clotting factors:
    • Factors II (Thrombin), VII, IX, and X

Indications

  • Long-term prevention of:

Side Effects

  • Bleeding
    • Antidote = Vitamin K
  • Teratogenic → Contraindicated in pregnancy
  • Narrow therapeutic index
  • Effectiveness highly variable due to:
    • Diet (vitamin K intake)
    • Other medications (especially CYP450 inducers like phenytoin, carbamazepine, phenobarbitone)
    • Liver function, alcohol use

Pharmacokinetics / Notes

  • Slow onset (2–3 days)
  • Requires INR monitoring
  • Suitable for long-term home use
  • Can be reversed with vitamin K or fresh frozen plasma
anticoagulant drugs: Coumadins
SteveKong3, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
anticoagulant drugs: Coumadins
Image credit: Rang, Dale, et al. Pharmacology; available from: https://amzn.to/3Hr51dO

Heparin-Induced Thrombocytopenia (HIT)

What Is It?

  • A reduction in platelet count due to heparin use

Types

  • Type I HIT
    • Within 1–2 days
    • Non-immune, transient, benign
  • Type II HIT
    • Day 5 onwards
    • Immune-mediated (IgG/IgM) against heparin–PF4 complex
    • Activates platelets → paradoxical thrombosis despite low platelet count

Summary – Anticoagulant Drugs

Anticoagulant drugs such as heparin, low molecular-weight heparins, and warfarin offer life-saving benefits in the management of thromboembolic disorders. Each class acts at a different point in the coagulation cascade and has unique pharmacological profiles, routes of administration, and monitoring requirements. Understanding these distinctions is vital for safe prescribing and patient safety. For more, see our Pharmacology & Toxicology Overview page.

Shopping Cart
Scroll to Top