drug excretion in milk

Breast milk is made from the mother's blood, so whatever is dissolved in her plasma can find its way into the milk. For a nursing infant this turns the mother's medication into a possible, if usually small, route of exposure. The amount that crosses over depends on the chemistry of the drug.

Most drugs reach milk by simple passive diffusion of the free, unbound, lipid-soluble fraction across the breast tissue. Several properties increase transfer: high lipid solubility, low molecular weight, low plasma protein binding, and being a weak base. Because milk is slightly more acidic than plasma, weak bases can become ionized and concentrate there by ion trapping, raising their milk-to-plasma ratio.

In practice the dose the infant actually receives is usually a tiny fraction of the maternal dose, and many drugs are considered compatible with breastfeeding. The judgement weighs the relative infant dose, the drug's safety profile and the infant's age and health. A few agents, such as some cytotoxic drugs, radioactive compounds and certain others, do warrant avoiding breastfeeding.