Principles of drug transfer into breast milk and drug disposition in the nursing infant.

There are numerous factors to consider when assessing the safety of drugs in lactating women. Drug properties facilitating transfer into milk as well as the pharmacokinetic properties of the drug in the mother and infant must be evaluated. Drug properties which promote low milk concentrations are: large volume of distribution, high protein binding, low lipid solubility, ionization at physiologic pH and large molecular weight. Following transfer into breast milk, drugs with low bioavailability and short elimination half-lives in neonates have improved safety.