PIP: The mechanism of drug transport from the maternal circulation into breast milk, and the effects of various drugs on the feeding infant are briefly reviewed. The transport of substances into breast milk from the mother's circulatory system depends on the lipid solubility of the drug, the pH of the milk, the pKa of the drug, and the protein binding of the drug. Concentration of the drug in maternal milk is a function of the volume of mammary alvoelar cells and the concentration of unbound drug in the mother's plasma, which depends on how her body handles the drugs's absorption and further metabolism. In general, drugs pass through membranes by passive diffusion dependent on lipid solubility and concentration gradient. Since milk pH (7) is slightly lower than plasma (7.4), weak bases tend to have high milk/plasma concentration ratios, and weak acids the reverse. Based on reports of adverse effects in infants, a number of drugs are contraindicated in nursing mothers. These include anticoagulants, antimetabolites, antithyroid agents, atropine, bromides, cathartics, ergot, iodides, mercurials, metronidazole, narcotics, and radioactive agents. Antibiotics should be administered with caution, but rarely produce acute effects in the infant. Alcohol and caffeine achieve low concentrations in maternal plasma, which are the same as those in maternal milk, but these substances have not been reported addictive or harmful to the infant.