Ethosuximide in human milk and in plasma of a mother and her nursed infant.

1 The concentrations of ethosuximide were measured in the milk and in the plasma of a nursing mother and her infant during a period of 4.5 months after delivery. 2 The maternal and the infant's plasma concentrations rose after delivery. 3 The milk concentration ethosuximide was similar to that in maternal plasma on the third day after delivery. During the following 2 months the average milk/maternal plasma concentration ratio was 0.80. The rise in ethosuximide concentration in milk during the first month was steeper than that in infant's plasma, which may be due to an increase in the infant's clearance of the drug. 4 Even if the infant has subtherapeutic plasma concentrations it is recommended to control the levels in infants that are nursed by ethosuximide-treated mothers when nursing has been established.