Campbell and Carter write that sulfamethoxazole should be listed in our systematic review as a drug that potentiates the anticoagulant effect of warfarin. The letter speaks to 3 of the persistent problems with the drug interaction literature and clinical practice.
The first problem is the lack of information on drug combination components’ individual effects. Although sulfamethoxazole should be suspected as an interacting drug given its metabolism, all of the literature that we could find (including the references provided by Campbell and Carter) studied or discussed co-trimoxazole, which is the sulfamethoxazole-trimethoprim combination also known as Bactrim or Septra. It is not clear to us which component, or both, interacts with warfarin.
Holbrook A, Pereira J. Comments on the Important Drug Interaction of Warfarin and Sulfamethoxazole—Reply. Arch Intern Med. 2005;165(21):2540. doi:10.1001/archinte.165.21.2540-b