Pharmacologic considerations of drug use in the lactating mother.

The recent increase in the incidence of breast-feeding has given impetus to the study of the excretion of drugs and chemicals into human milk. It appears that the major route of drug appearance in milk is via diffusion from the maternal circulation. In general, maternal plasma levels of a drug dictate milk levels. Un-ionized drugs with high lipid solubility and minimal binding to maternal plasma protein diffuse best. The amount of a drug excreted in milk is usually not more than 1 to 2% of the maternal dose. Most studies have been done with single-dose or short-term drug administration. Very few data are available for the mother who receives continuous drug therapy. Environmental chemicals such as insecticides are also a cause for special concern. They are highly lipid soluble and may remain in body fat for very long periods. Indeed, lactation may be the only route of elimination. The effect of even small amounts of these agents on the growing infant is unknown. Further studies are needed to determine the amount of these agents secreted and the possible risk to the nursing infant.