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
Drug excretion into the milk depends on a number of factors related to the
drug including (
- 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
- The pH of the plasma and the milk -
Weakly alkaline drugs have higher breast milk levels than weak acids.
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
This mechanism concentrates the drug in the breast milk.
After diffusion or active transport, drugs pass through spaces between
alveolar cells into the milk (
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 (