Q; 52 year old female with history of hypertension - but controlled with Lisinopril developed UTI (urinary tract infection). What should be your worry to treat her with Bactrim (TMP-SMX)?



Answer: Hyperkalemia

The mechanism of Bactrim induced hyperkalemia is via Trimethoprim inhibition of the sodium channel located on the luminal surface of the principal cells. This decreases sodium reabsorption and the electro negativity in the lumen thus decreases the driving force for the secretion of potassium through an apically located potassium channel on the same principal cells.

This side effect is most common in HIV infected patients who are treated with high doses of TMP-SMX. However hyperkalemia can occur with lower doses in patients with preexisting renal dysfunction or in those taking medications such as angiotensin-converting enzyme inhibitors or potassium-sparing diuretics. It may exacerbate hyperkalemic effect to potentially dangerous levels.