Mechanisms of drug excretion in human milk include both passive diffusion and carrier-mediated transport. The amount of drug excreted in milk depends on the drug’s molecular weight, lipid solubility, pKa, and plasma protein binding.146,147 Once in milk, the pKa of the drug is an important determinant of the drug’s concentration in milk. Consequently, at equilibrium some drugs may accumulate in milk in higher concentration relative to plasma.
The penicillins are excreted in milk and may lead to sensitization of infants.38,39 Metronidazole, which has been shown to be carcinogenic in rats and mice, is excreted in milk in concentrations similar to those found in plasma.61 Clindamycin is also excreted in 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.38,39,61,70,73
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