The issue of medication use during pregnancy is of concern because the physiology of pregnancy affects the pharmokinetics of medications used, and certain medications can reach the fetus and cause harm. Studying medication safety in pregnancy and lactation is challenging; thus, the U.S. Food and Drug Administration (FDA) categories of medication risk in pregnancy are limited, especially for the lactating mother. A better understanding of the role of physiologic changes in pregnancy, placental function, effects of medication on the fetus, and the mechanisms of drug transfer into breast milk can help nurses teach their patients both preconceptionally and during pregnancy and lactation. This article provides a review of current literature so nurses can become more aware of the basic principles involved in medication use for pregnant and lactating women.