<p>Elderly patients, due to the decline of physiological functions, coexistence of multiple diseases, and physiological decline in liver and kidney functions, experience significant changes in the pharmacokinetics and pharmacodynamics of antibacterial drugs, with a markedly increased risk of adverse drug reactions. This article reports a case of a 91-year-old male patient who, due to a pulmonary infection and hip replacement surgery, successively used four β-lactam antibacterial drugs: ceftriaxone, cefoperazone-sulbactam, meropenem, and piperacillin-tazobactam. On the third day after switching to cefoperazone-sulbactam, the patient developed a delayed generalized rash (Naranjo score 6); on the fifth day of meropenem administration, delirium occurred (Naranjo score 6), which resolved after discontinuation of the drug; within 5&#xa0;min of infusion of piperacillin-tazobactam, a rapid-onset generalized allergic reaction occurred (Naranjo score 7), suggesting cross-allergy mediated by homologous side-chain structures of the drugs. After multiple consultations with clinical pharmacists, and with timely discontinuation of the drugs and symptomatic treatment by the clinicians, all the patient's symptoms were relieved. This case indicates that elderly patients require strengthened individualized medication monitoring and high vigilance for cross-allergic reactions caused by homologous side-chain structures of β-lactam drugs. Full-process pharmaceutical supervision is key to ensuring the safe use of antibacterial drugs in elderly patients.</p>