Ingested Magnesium does not concentrate into breast milk.. Naturally occurring, the mean Magnesium concentration in the milk is 31 mg/L (range 15 – 64 mg/L) and not affected by the ingestion of Magnesium. Because of a low oral bioavailability the pass from the breast milk toward the infant's plasma is hampered, except in premature and newborn infants who may exhibit a higher intestinal absorption due to an increased permeability. Avoid chronic or excessive use. WHO Model List of Essential Medicines 2002: Magnesium hydroxide is compatible with breastfeeding.