Just happened upon this while doing a Medline search ....

Br J Clin Pharmacol 1997 Dec;44(6):543-547

Methadone distribution and excretion into breast milk of clients in a methadone
maintenance programme.

Wojnar-Horton RE, Kristensen JH, Yapp P, Ilett KF, Dusci LJ, Hackett LP

[Medline record in process]

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.