Mechanisms of drug excretion in human breast milk include both passive diffusion and carrier-mediated transport. The amount of drug excreted in breast milk depends on the drug’s molecular weight, lipid solubility, pKa, and plasma protein binding.150,151 Once in breast milk, the pKa of the drug is an important determinant of the drug’s concentration in breast milk. Consequently, at equilibrium some drugs may accumulate in breast milk in higher concentration relative to plasma.
The penicillins are excreted in breast milk and may lead to sensitization of infants.34,37 Metronidazole, which has been shown to be carcinogenic in rats and mice, is excreted in breast milk in concentrations similar to those found in plasma.61 Clindamycin is also excreted in breast milk.70 The fate of azithromycin is unknown.73 Considering the potential risks to the nursing infant and benefits to the mother, a decision should be made whether to discontinue nursing or not to prescribe an antibacterial agent.34,37,61,70,77
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