BACKGROUND: Patients with low body weight (LBW) often exhibit altered pharmacokinetics (PK) in renal clearance and total body water. These changes complicate β-lactam antibiotic dosing, potentially resulting in suboptimal efficacy or increased toxicity. OBJECTIVES: To evaluate the attainment of PK/pharmacodynamic (PD) targets, the prevalence of subtherapeutic and supratherapeutic concentrations, and the incidence of neurotoxicity among LBW patients treated with piperacillin/tazobactam (TZP), cefepime (FEP), and meropenem (MEM). DESIGN: A prospective observational study conducted at a tertiary hospital from January 2020 to December 2022. METHODS: Adult patients with a body mass index ⩽18.5 kg/m RESULTS: Seventy-two patients were included: 29 received TZP, 23 FEP, and 20 MEM. Achievement of the 100% fT >
MIC target was comparable across all antibiotics (~70%), but 100% fT >
4 MIC attainment was significantly higher for FEP (47.8%) than for TZP (10.3%) and MEM (30%) ( CONCLUSION: PK changes complicate β-lactam antibiotic dosing, resulting in frequent failure to achieve PK/PD targets. FEP demonstrated a particularly high risk of supratherapeutic concentrations and neurotoxicity. Therapeutic drug monitoring is crucial to optimize dosing and improve safety in this population.