Drugs that increase the anticoagulant effect of warfarin without affecting its plasma levels include the second-and third-generation cephalosporins, which augment the anticoagulant effect of warfarin by inhibiting the cyclic interconversion of vitamin K18; clofibrate, Requests for reprints should be sent to the Office of Scientific Affairs, American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231-4596. at NORTH DAKOTA STATE UNIV on June 26, 2015 http://circ.ahajournals.org/ Downloaded from pregnancy. Chest. 1992;102(suppl):385S-390S. at NORTH DAKOTA STATE UNIV on June 26, 2015 http://circ.ahajournals.org/ Downloaded from J Hirsh and V Fuster Guide to anticoagulant therapy.