The use of drugs interfering with the rennin-angiotensin system has become wider and wider, in recent years, with the marketing of ACE-inhibitors before and of ARB (sartans) afterwards. With such a high number it is easy to discover the birth of new interactions between drugs. Knowing that hyperkaliemia has been associated with the use of ACE-inhibitors and trimethoprim, a case-control trial has been performed in Toronto among subjects ≥ 66 years of age under continuative treatment with drugs inhibiting rennin-angiotensin system and administered with an antibiotic. Cases (371 patients hospitalized for hyperkaliemia who had taken an antibiotic within 14 days before) have been compared with 1-4 controls in similar clinical conditions but without hospitalization. The risk of hyperkaliemia has been about 7 times higher with trimethoprim-sulfamethoxazole than with amoxicillin (OR 6.7; 95% IC 4.5-10.0). Even when hospitalization has been made within 7 days from antibiotic use, they have had the same results. There has been, instead, no link between hyperkaliemia and other used antibiotics (chinolonics, nitrofurantoin).
In old patients under treatment with ACE-inhibitors or sartans, the use of trimethoprim-sulfamethoxazole should not be prescribed, if there is a practicable alternative.
