Captopril × Co-trimoxazole

Trimethoprim (a co-trimoxazole component) blocks sodium channels in the collecting duct epithelium (amiloride-like mechanism) and reduces potassium secretion. Captopril (ACE-I) additionally retains potassium via aldosterone suppression.

Muscle weakness, paraesthesia in the limbs, slowed pulse, arrhythmias. ECG shows peaked T waves and widened QRS. Severe cases progress to cardiac arrest. Symptoms typically appear within 1–2 weeks of starting the combination.

During co-trimoxazole therapy (usually 5–10 days), check potassium 3–5 days after start. In older patients and chronic kidney disease, prefer alternative antibiotics: nitrofurantoin or fosfomycin for urinary tract infection.