We wish to thank Dr Young for his interest in our Motherisk Update, and for his thoughtful observations.
The dose of 87 μg/L of milk calculated by him is not “such a small quantity” for a newborn. In fact, it is 30 μg/kg. In older infants a dose of 50 μg/kg is used for sedation. The newborn has much lower capacity to deactivate morphine.1 Moreover, the newborn has substantially higher sensitivity to the central effects of morphine, partially due to more penetration through the blood-brain barrier.2
Last, as we indicated in the paper, the homozygocity the child exhibited to glucuronidation of morphine might be associated with higher levels of the morphine-6-glucuronide metabolite, which is many times more active than morphine. We have not measured this metabolite in milk, but we are now in the process of doing so.
With research support from Genome Canada, we are currently studying the prevalence and risk of codeine use while breastfeeding.
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