Drug therapy during lactation requires recognition of several elements, including drug properties, the status of the infant, the mother's milk production, and what is known about particular drugs. It is important to consider the various properties of a drug when evaluating its possible effects in the breastfeeding infant. In addition, the volume of milk the infant receives and its composition can influence exposure levels. Other factors of importance include the infant's age, frequency of feedings, recommended drug dosage, and whether the drug is short- or long-acting. Suggestions are offered for appropriate drug therapy when the mother is breastfeeding, including the safety of drugs commonly administered during lactation and questions to ask when a mother seeks assistance about suggested medication use.