RESUMEN
Two patients, a girl and a boy, both aged 8.5 years, presented with serious side effects caused by ceftriaxone and co-trimoxazole, respectively. The first patientwas treated with ceftriaxone (100 mg/kg/day with a body weight of 35.6 kg) on suspicion of a neuroborreliosis, but developed an acute cholecystitis with cholelithiasis 3 weeks after the antibiotic had been withdrawn. He underwent a laparoscopic cholecystectomy. Ceftriaxone binds calcium in the biliary tract, forming biliary sludge or stones. The second patient developed thrombocytopenia during treatment with co-trimoxazole (58 mg/kg/day with a body weight of 25.4 kg) because of a urinary-tract infection. After discontinuation of the co-trimoxazole the thrombocytopenia resolved spontaneously. The pathophysiological mechanism involved may be either a direct toxic effect of trimethoprim or an immune-mediated reaction to sulfamethoxazole. According to current guidelines, the dosage of the drug was too high in both cases. It is important to ensure a correct dosage in children, since side effects are potentially dose-related.