We would like to comment on the article by Tan et al.¹ The purpose of measuring serum levels of a drug is either to monitor the toxicity of the drug or the therapeutic concentration for a particular condition. Emergence of infections with β-lactam-resistant Staphylococcus epidermidis, Staphylococcus aureus, and Enterococcus sp, has led to the frequent use of vancomycin in neonates. Vancomycin has historically had a reputation for toxicity. Many of its original adverse reactions, including ototoxicity and nephrotoxicity, were probably due impurities in the formulation.² Now that a more purified form is available, these adverse reactions are uncommon. However, concomitant administration with aminoglycosides or other nephrotoxins may increase the risk of toxicity.³ Effective drug therapy is measured by response, not by achievement of a particular circulating drug concentration. Because the association between vancomycin peak concentrations and toxicity is poor, some have recommended measuring trough concentration only⁴ as this study is clearly documenting, but others have suggested not measuring any concentrations in the majority of children with normal renal function.⁵ However, in critically ill premature neonates with poor glomerular filtration rate, prematurity, and compromised cardiovascular function, it remains prudent to measure both peak and trough concentrations in those with poor or changing renal function. Caution must be exercised when other nephrotoxic or ototoxic drugs such as aminoglycisides are administered concurrently.⁶ In this study, the authors did not mentioned the concomittent use of aminoglycoside.