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The epithelium of the breast milk alveolar cells is most permeable
to drugs during the first week post-partum,
so drug transfer to milk may be greater during the first week of an
infant's life.
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Drug excretion into the milk depends on a number of factors related to the
drug including (
Lawrence, 2011
p. 365-369):
- Ionization of the drug - Drugs that are not protein bound and
nonionized are more likely to be transferred into breast milk.
- Molecular weight of the drug - Lower molecular weight drugs are more
likely to be transferred to the breast milk than
higher molecular weight drugs.
- Solubility of the drug in lipids and waters -
Lipid soluble drugs pass more freely into breast milk than water
soluble drugs.
- The pH of the plasma and the milk -
Weakly alkaline drugs have higher breast milk levels than weak acids.
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Once the drugs have reached the alveolar cells of the breast
they may then be transferred into the milk by:
- Diffusion: the movement of the drug from a high
concentration area (blood) to a low concentration area (breast milk).
- Active transport: the movement of the drug from
blood with a low concentration to breast milk with a high
concentration.
This mechanism concentrates the drug in the breast milk.
After diffusion or active transport, drugs pass through spaces between
alveolar cells into the milk (
Lawrence, 2011
p. 369).
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Most ingested drugs that appear in the milk do not exceed 2%
of the ingested dose and the binding of the drug to milk proteins
is less than the binding to plasma proteins (
Powers, 1997
).
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