Dr Horn is indeed correct that most of the alerts in our study were triggered by warfarin being prescribed to patients who already were receiving trimethoprim-sulfamethoxazole. Some of this was due to warfarin being prescribed 3 times more commonly than trimethoprim-sulfamethoxazole, as noted in our article.1 Dr Horn points out that dose titration of the warfarin might have allowed it to be used safely in patients already receiving trimethoprim-sulfamethoxazole. However, such patients might then be at risk of underanticoagulation when stopping therapy with trimethoprim-sulfamethoxazole, in most cases after being discharged from the hospital.
Strom BL, Schinnar R, Aberra F, Bilker W, Hennessy S, Leonard CE. Hard-Stops for Drug Interactions—Reply. Arch Intern Med. 2011;171(7):702-710. doi:10.1001/archinternmed.2011.110