Background/Objectives: The combination drug sulfamethoxazole/trimethoprim (ST) is a broad-spectrum antibiotic used against various infections; however, it is associated with several serious adverse events. The ST package inserts contain warnings about these adverse events. However, warnings vary internationally, and specific measures to address ST-related adverse events are unclear. Therefore, we aimed to comprehensively evaluate ST-related adverse events using the Japanese Adverse Drug Event Report (JADER) database and analyze the onset time for each event. Methods: Adverse events due to ST were analyzed using the JADER database between April 2004 and June 2023. The reported odds ratio and 95% confidence interval (95% confidence interval [CI]) were calculated, with a signal detected if the 95% CI lower limit exceeded 1. The Weibull distribution was used to characterize the onset time of adverse events with detected signals. Results: The total number of cases in the JADER database during the study period was 862,952, and the number of adverse events involving ST as a suspected drug was 4203. Adverse events associated with ST include hyperkalemia, syndrome of inappropriate antidiuretic hormone secretion, hematopoietic cytopenia, acute renal failure, hypoglycemia, disseminated intravascular coagulation syndrome, hepatic disorder, and the Stevens–Johnson syndrome/toxic epidermal necrolysis. Conclusions: Weibull analysis indicated an early failure-type onset time for all adverse events, suggesting the need for intensive adverse event monitoring of ST, especially in the first month of use. These findings may support revising drug package inserts in Japan to better reflect the identified risks.

Infections caused by extended-spectrum β-lactamase-producing Enterobacterales (ESBL-Es) pose a significant global threat with notable increases in prevalence worldwide. Carbapenems are often used as the first line of treatment. However, their overuse accelerates resistance development, highlighting the urgent need for clinically viable carbapenem-sparing strategies. Cefmetazole (CMZ) and flomoxef (FMOX) are parenteral antibiotics that are widely used in Japan and have emerged as potential carbapenem alternatives. Repositioning these agents effectively addresses the clinical need for carbapenem-sparing strategies and outpatient ESBL-E management. This review aims to reposition CMZ and FMOX for real-world clinical practice by synthesizing basic research, clinical studies, and pharmacokinetics/pharmacodynamics (PKs/PDs) analyses, which suggest that these agents may be effective in treating ESBL-E infections—particularly urinary tract infections, as evidenced by their minimum inhibitory concentration (MIC) values. The clinical outcomes of these interventions have been comparable to those of carbapenems, which support their role in antimicrobial stewardship. Their PK/PD characteristics emphasize the importance of dose optimization to ensure therapeutic efficacy, whereas recent insights into resistance mechanisms provide a foundation for appropriate use. As novel antibiotic development takes substantial time, revisiting existing options is increasingly important. Notably, the Infectious Diseases Society of America’s 2024 guidance on antimicrobial resistance has omitted CMZ and FMOX, owing to which clinicians have limited guidance on their use, particularly in regions like Japan where these antibiotics are widely employed. By addressing this knowledge gap, the present review offers a comprehensive evaluation of these drugs and highlights their potential as intravenous agents in ESBL-E management. Furthermore, it highlights the ongoing challenge of ensuring effective oral step-down therapy in an outpatient setting to reinforce the global relevance of CMZ and FMOX in a broader treatment framework, underscoring their potential for outpatient administration where clinically appropriate.