Peak milk levels occurred 4 to 6 hours after a dose. There was little difference in milk levels before and after feeding. Her colostrum methadone level was 77 mcg/L on the day of delivery and milk levels remained fairly constant (range 77 to 123 mcg/L) throughout the study period. One lactating mother taking oral methadone maintenance 45 mg daily had milk sampled several times over the first 4 weeks postpartum and again several times over 24 hours at 6 weeks postpartum. Milk levels ranged from 110 to 180 mcg/L in the first mother and 110 to 250 mcg/L in the second.
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Two postpartum mothers taking oral methadone maintenance, one 73 mg once daily and the other 30 mg twice daily, had serial milk levels sampled on day 11 and 14 postpartum, respectively. Trough levels were about 15 mcg/L on both days. The peak methadone milk level was 700 mcg/L on day 5 and 600 mcg/L on day 6, both occurring 3 hours after the dose. One mother taking 25 mg daily of oral methadone maintenance had breastmilk sampled twice on postpartum days 5 and 6. Using the peak level from this study, an exclusively breastfed infant would receive a maximum of 86 mcg/kg daily of methadone, equal to about 6% of the maternal weight-adjusted dosage. Ten mothers who were 3 to 10 days postpartum and taking oral methadone maintenance 10 to 80 mg once daily had single milk samples that contained methadone levels ranging from 50 to 570 mcg/L. Using the peak milk level from this study, an exclusively breastfed infant would receive 18 mcg/kg daily, equal to about 2% of the maternal weight-adjusted dosage. Lower milk levels (20 and 30 mcg/L) were measured 0 to 3 hours after a dose. At 6 and 15 hours after a dose, the level was 40 mcg/L. The highest methadone breastmilk levels (100 and 120 mcg/L) were measured 4 hours after a dose. One mother taking 50 mg daily of oral methadone maintenance had milk sampled several times on postpartum days 4 through 8. In adults, methadone oral bioavailability is 80 to 95%.
Methadone is metabolized to inactive pyrrolidine and pyrroline metabolites. The breastfeeding rate among mothers taking methadone for opiate dependency has been lower than in mothers not using methadone in some studies, but this finding appears to vary by institution, indicating that other factors may be important. Abrupt weaning of breastfed infants of women on methadone maintenance might result in precipitation of or an increase in infant withdrawal symptoms, and gradual weaning is advised.
The long-term outcome of infants breastfed during maternal methadone therapy for opiate abuse has not been well studied.
Some studies have found shorter hospital stays, durations of neonatal abstinence therapy and shorter durations of therapy among breastfed infants, although the dosage of opiates used for neonatal abstinence may not be reduced. Breastfeeding may decrease, but not eliminate, neonatal withdrawal symptoms in infants who were exposed in utero. Women who received methadone maintenance during pregnancy and are stable should be encouraged to breastfeed their infants postpartum, unless there is another contraindication, such as use of street drugs. Other agents are preferred over methadone for pain control during breastfeeding. If the baby shows signs of increased sleepiness (more than usual), breathing difficulties, or limpness, a physician should be contacted immediately. Initiation of methadone postpartum or increasing the maternal dosage to greater than 100 mg daily therapeutically or by abuse while breastfeeding poses a risk of sedation and respiratory depression in the breastfed infant, especially if the infant was not exposed to methadone in utero. Most infants receive an estimated dose of methadone ranging from 1 to 3% of the mother's weight-adjusted methadone dosage with a few receiving 5 to 6%, which is less than the dosage used for treating neonatal abstinence.
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