What is a known electrolyte side effect of trimethoprin use?

• A. Hyponatremia
• B. Hypomagnesemia
• C. Hyperkalemia
• D. Hypercalcemia

Answer: (C)

Trimethoprim can cause hyperkalemia by acting like a potassium-sparing agent in the kidney. It blocks channels in the distal nephron (the epithelial sodium channels), which reduces potassium secretion into the urine. As a result, serum potassium can rise, especially in people with reduced kidney function, older adults, or those taking other medications that raise potassium (like ACE inhibitors, ARBs, or potassium supplements).

This is the main electrolyte concern with trimethoprim. Hyponatremia, hypomagnesemia, and hypercalcemia are not typical, direct effects of trimethoprim, so they’re not expected as common side effects in this context. Monitoring potassium and kidney function is important when starting trimethoprim or when it’s used with other potassium-influencing drugs.