Disposition of methadone and its relationship to severity of withdrawal in the newborn.

We studied the placental transfer of methadone, the relationship of neonatal plasma methadone concentrations to withdrawal symptomatology, and the relationship between maternal methadone dose and severity of neonatal withdrawal in 31 methadone-maintained mothers and their neonates. Methadone concentrations in maternal, cord and neonatal plasma were measured using a gas chromatographic micromethod. Neonatal plasma was assayed on days 0-5 of life. Urine methadone levels were measured for the first 3 days of neonatal life, using a similar assay. Twenty-five of the neonates experienced mild to severe withdrawal symptoms. There was no consistent relationship between the maternal methadone dose and the severity of neonatal symptoms. However, when neonatal withdrawal did occur, it began after plasma methadone levels fell below .06 mug/ml. The neonatal plasma methadone levels were consistently lower than those of the mother. Maternal methadone is transferred across the placenta and can induce significant withdrawal symptomatology in the newborn.