Anticoagulant

An anticoagulant is a drug that reduces the blood's tendency to clot by interfering with the coagulation cascade. Anticoagulants do not dissolve existing clots; they prevent the formation of new thrombi and limit growth of existing ones. They are used to prevent stroke in nonvalvular atrial fibrillation, treat and prevent venous thromboembolism, and reduce risk after certain surgeries. They come in several classes with different mechanisms and routes of administration.

Indirect thrombin inhibitors, such as unfractionated heparin and low-molecular-weight heparins (enoxaparin), enhance antithrombin activity and inhibit thrombin and factor Xa. They are usually given by injection and require monitoring (unfractionated heparin with aPTT; LMWH with weight-based dosing). Reversal is with protamine.

Vitamin K antagonists, most commonly warfarin, are oral agents that reduce synthesis of vitamin K–dependent clotting factors. They require regular monitoring of the international normalized ratio (INR) and are affected by diet and drug interactions. Warfarin is contraindicated in pregnancy.

Direct-acting anticoagulants include direct oral anticoagulants (DOACs): factor Xa inhibitors (rivaroxaban, apixaban, edoxaban) and the direct thrombin inhibitor dabigatran. DOACs have more predictable effects and do not require routine INR monitoring; they are preferred for nonvalvular atrial fibrillation and many cases of DVT/PE, though they are not used for mechanical heart valves.

Reversal strategies vary: vitamin K and fresh frozen plasma or PCC for warfarin; andexanet alfa or 4-factor PCC for factor Xa inhibitors; idarucizumab for dabigatran. Bleeding risk, renal and hepatic function, pregnancy, and interactions with other drugs influence choice and dosing.