Inter- and intrapatient variability in excretion of phenobarbital into breastmilk is extensive. Phenobarbital in breastmilk apparently can decrease withdrawal symptoms in infants who were exposed in utero, but it can also cause drowsiness in some infants, especially when used with other sedating drugs. Monitor the infant for drowsiness, adequate weight gain, and developmental milestones, especially in younger, exclusively breastfed infants and when using combinations of psychotropic drugs. Sometimes breastfeeding might have to be limited or discontinued because of excessive drowsiness and poor weight gain. If there is concern, measurement of the infant's serum phenobarbital concentration might help rule out toxicity.