Abstract Neonates administered ethanol-containing medicines are potentially at risk of dose-dependent injury through exposure to ethanol and its metabolite, acetaldehyde. Here, we determine blood ethanol and acetaldehyde concentrations in 49 preterm infants (median birth weight = 1190 g) dosed with iron or furosemide, medicines that contain different amounts of ethanol, and in 11 control group infants (median birth weight = 1920 g) who were not on any medications. Median ethanol concentrations in neonates administered iron or furosemide were 0.33 (range = 0–4.92) mg/L, 0.39 (range = 0–72.77) mg/L and in control group infants were 0.15 (range = 0.03–5.4) mg/L. Median acetaldehyde concentrations in neonates administered iron or furosemide were 0.16 (range = 0–8.89) mg/L, 0.21 (range = 0–2.43) mg/L and in control group infants were 0.01 (range = 0–0.14) mg/L. There was no discernible relationship between blood ethanol or acetaldehyde concentrations and time after medication dose. Conclusion: Although infants dosed with iron or furosemide had low blood ethanol concentrations, blood acetaldehyde

concentrations were consistent with moderate alcohol exposure. The data suggest the need to account for the effects of acetaldehyde in the benefit-risk analysis of administering ethanol-containing medicines to neonates.