Question

a nurse is caring for a client who is taking warfarin and has a new prescription for trimethoprim/sulfamethoxazole to treat a urinary tract infection. the nurse should clarify the prescriptions with the provider because taking these two medications concurrently can increase the client’s risk for which of the following?

• bleeding
• thrombosis
• ecg changes
• ototoxicity

Explanation:

Warfarin is an anticoagulant that inhibits vitamin - K dependent clotting factors. Trimethoprim/sulfamethoxazole (TMP/SMX) can displace warfarin from protein - binding sites and also inhibit the metabolism of warfarin by cytochrome P450 enzymes (specifically CYP2C9). This leads to an increased plasma concentration of free (unbound) warfarin. An increased level of warfarin enhances its anticoagulant effect, which in turn increases the risk of bleeding.
Thrombosis is the opposite of what would be expected as warfarin's main action is to prevent thrombosis (by inhibiting clotting). TMP/SMX does not have a mechanism that would counteract warfarin's anticoagulant effect to cause thrombosis.
ECG changes are not a typical interaction between warfarin and TMP/SMX. Warfarin's main effect is on the coagulation cascade, and TMP/SMX's main effects are antibacterial (by inhibiting folic acid synthesis in bacteria) and some potential effects on the immune system (rarely) but not on the electrical conduction of the heart (which would cause ECG changes).
Ototoxicity is usually associated with drugs like aminoglycosides (by damaging the hair cells in the inner ear), loop diuretics (in high doses), etc. There is no known mechanism by which the combination of warfarin and TMP/SMX would cause ototoxicity.

Answer:

Bleeding