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Phenobarbital crosses the placenta quickly, and the balance between maternal and fetal blood is achieved in a few minutes. Data on the clinical outcomes of infants born to mothers under phenobarbital treatment during pregnancy show that they are at risk of adverse events, such as sedation and abstinence syndrome. The aim of this study was to analyse the correlation between serum levels of phenobarbital and clinical features of neonates.Twenty-three infants born between 2001 and 2008 were studied. Maternal, neonatal and pharmacological variables were considered.Eleven infants displayed symptoms related to phenobarbital. Withdrawal syndrome was seen in seven infants and sedation syndrome was seen in four infants. One infant had severe cardiorespiratory depression at birth. None of the infants had severe neonatal abstinence syndrome. No statistically significant differences were found between symptomatic and asymptomatic infants. At birth, the mean serum level of phenobarbital of the 23 infants was 15.4 [standard deviation (SD) 6.2] μg/ml. A peak (16.1 μg/ml, SD 5.5) was seen on Day 3, followed by a gradual decrease to non-therapeutic levels (10 μg/ml) by Day 8 (9.3 μg/ml, SD 1.0). Phenobarbital levels were higher in symptomatic infants than asymptomatic infants, although the difference was not statistically significant.Serum levels of phenobarbital remained in the therapeutic range for both mothers and infants, and reduced gradually in infants. However, some infants displayed symptoms related to phenobarbital. As such, a clinical pharmacological surveillance protocol is necessary.