What is a common adverse effect of TMP-SMX other than hypersensitivity, and what patient population requires caution?

• A. Hyperkalemia; caution in renal impairment and pregnancy.
• B. Hypokalemia; caution in liver disease.
• C. Hyponatremia; caution in elderly patients.
• D. Hyperglycemia; caution in diabetics.

Answer: (A)

Hyperkalemia is a common adverse effect of trimethoprim-sulfamethoxazole because the trimethoprim part blocks epithelial sodium channels in the distal tubule, which reduces potassium excretion in the urine. This electrolyte effect is distinct from hypersensitivity and is a well-recognized risk with this antibiotic.

Because potassium balance depends on kidney function, reduced renal function heightens the risk of dangerous hyperkalemia, so caution or dose adjustment is needed in renal impairment. Pregnancy also requires caution with TMP-SMX due to safety concerns for the fetus (folate antagonism and other obstetric considerations). Together, these factors explain why hyperkalemia with caution in renal impairment and pregnancy is the best answer.