Co-trimoxazole × Valsartan

Trimethoprim blocks collecting duct sodium channels (amiloride-like mechanism) and reduces potassium secretion. Valsartan (ARB) additionally retains potassium through 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, check potassium 3–5 days after start. In older patients and chronic kidney disease, prefer alternative antibiotics: nitrofurantoin or fosfomycin.