Identify the drug that takes at least 5 days to become a fully active anticoagulant.

• A. Dabigatran
• B. Warfarin
• C. Heparin
• D. Rivaroxaban

Answer: (B)

Onset timing for anticoagulants varies, and warfarin has a notably delayed onset. It works by inhibiting vitamin K–dependent synthesis of clotting factors II, VII, IX, and X, plus proteins C and S. Because it reduces the production of these factors rather than affecting existing ones immediately, the full anticoagulant effect doesn’t appear until those factors have turned over, which typically takes about 4–5 days or longer. This is why you often bridge with heparin or LMWH to provide immediate anticoagulation while warfarin’s effect ramps up. In contrast, dabigatran and rivaroxaban are direct oral anticoagulants with rapid onset within hours, and heparin acts immediately by enhancing antithrombin to inhibit thrombin and factor Xa.