Major
Co-trimoxazole × Losartan
Sulfonamide-trimethoprim combination×Angiotensin II receptor blockers (sartans)
Mechanism
Trimethoprim blocks sodium channels via an amiloride-like mechanism, additively with an ARB (losartan) causing hyperkalaemia. In older patients, sudden death risk rises.
Symptoms
Muscle weakness, paraesthesia, slowed pulse, arrhythmias. ECG: peaked T waves, widened QRS. Severe cases progress to cardiac arrest.
Management
During co-trimoxazole therapy, check potassium 3 days after start. In older patients and chronic kidney disease, prefer alternative antibiotics (nitrofurantoin, fosfomycin).