An 81-year-old woman with T2DM maintained on glipizide (Glucotrol) 10 mg daily was started 3 days ago on trimethoprim-sulfamethoxazole (Bactrim DS) for a urinary tract infection. She now presents to clinic confused, diaphoretic, and tremulous. Fingerstick glucose is 42 mg/dL. Which of the following best explains this presentation?

Question

Accepted Answer

✅ C. TMP-SMX potentiates sulfonylurea-induced hypoglycemia through CYP2C9 inhibition and displacement of glipizide from plasma protein binding

Trimethoprim-sulfamethoxazole inhibits CYP2C9, the primary metabolic pathway for sulfonylureas (glipizide, glimepiride, glyburide). This reduces glipizide clearance, elevates plasma levels, and causes hypoglycemia — a well-documented and common outcome, particularly in elderly patients with reduced hepatic and renal reserve. Sulfamethoxazole also displaces sulfonylureas from albumin, further increasing free drug. Other CYP2C9 inhibitors to watch: fluconazole, amiodarone, metronidazole.

❌ A. TMP-SMX-induced acute kidney injury has accumulated her glipizide, causing hypoglycemia
While TMP-SMX can cause AKI and hyperkalemia, and reduced renal clearance does prolong sulfonylurea effect, the primary and classic mechanism here is CYP2C9 inhibition plus protein binding displacement, not AKI alone.

❌ B. TMP-SMX-induced hyperkalemia has triggered insulin-mediated glucose uptake
TMP-SMX does cause hyperkalemia (trimethoprim acts like amiloride on ENaC), but this does not cause hypoglycemia. The two adverse effects are independent.

❌ D. Sulfonylurea autoimmunity triggered by sulfa cross-reactivity
There is no autoimmune or anti-insulin antibody mechanism here. Sulfonylurea/sulfa cross-reactivity concerns are largely disproven clinically and would present as rash/allergic reaction, not hypoglycemia.

:::danger
Watch for sulfonylurea + CYP2C9 inhibitor combos (Bactrim, fluconazole, amiodarone, metronidazole) — severe hypoglycemia, especially in elderly. Per Beers Criteria, glyburide is to be avoided in older adults; glipizide is the preferred sulfonylurea when one must be used but still carries hypoglycemia risk.
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