Background <p>Ertapenem, a broad-spectrum carbapenem antibiotic, is commonly used for treating infections such as septic shock. However, its neurotoxic side effects, especially in elderly patients with multiple comorbidities and renal impairment, are rarely reported. This case report highlights a rare but clinically significant adverse effect of ertapenem—delirium—underscoring its importance in medical literature.</p> Case presentation <p>We report the case of a 76-year-old Chinese male patient admitted with septic shock secondary to community-acquired pneumonia. Initial treatment with ertapenem showed improvement, but the patient developed delirium on the third day of therapy. Comprehensive diagnostic evaluations ruled out common causes of cognitive impairment, leading to the suspicion of ertapenem-induced neurotoxicity. Upon discontinuation of the drug, the patient’s mental state gradually improved, with full cognitive recovery within days. The clear temporal association between ertapenem administration and the onset of delirium, alongside the rapid improvement after discontinuation, strongly supports a causal link.</p> Conclusion <p>This case underscores the importance of recognizing drug-induced delirium in elderly patients receiving ertapenem, particularly those with renal dysfunction or multiple comorbidities. By reporting this case, we aim to raise awareness of this rare but significant adverse effect, emphasizing the need for vigilance in monitoring neuropsychiatric symptoms in high-risk populations.</p>