Aims: Methadone is widely used in maintenance programs for opioid-dependent subjects. The aims of the study were to quantify the distribution and excretion of methadone in human milk during the early postnatal period and to investigate exposure of breast fed infants to the drug.
Methods: Blood and milk samples were obtained from 12 breast feeding women who were taking methadone in daily doses ranging from 20-80 mg (0.3-1.14 mg kg-1). Blood was also obtained from eight of their infants. Methadone concentration in these samples was quantified by h.p.l.c. The infants were observed for withdrawal symptoms.
Results: The mean (95% CI) milk/plasma ratio was 0.44 (0.24-0.64). Exposure of the infants, calculated assuming an average milk intake of 0.15 l kg-1 day-1 and a bioavailability of 100% was 17.4 (10.8-24) microg kg-1 day-1. The mean infant dose expressed as a percentage of the maternal dose was 2.79 (2.07-3.51)%. Methadone concentrations in seven infants were below the limit of detection for the h.p.l.c. assay procedure, while one infant had a plasma methadone concentration of 6.5 microg l-1. Infant exposure to methadone via human milk was insufficient to prevent the development of a neonatal abstinence syndrome which was seen in seven (64%) infants. No adverse effects attributable to methadone in milk were seen.
Conclusions: We conclude that exposure of breast fed infants to methadone taken by their mothers is minimal and that women in methadone maintenance programs should not be discouraged from breast feeding because of this exposure.