Major
Co-trimoxazole × telmisartan
Sulfonamide-trimethoprim combination×Angiotensin II receptor blocker (ARB)
Mechanism
Trimethoprim in co-trimoxazole blocks the epithelial sodium channel (ENaC) like amiloride – potassium retention. With an ARB (telmisartan): additive hyperkalaemia.
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).