Drug use during pregnancy and lactation remain underdeveloped areas of clinical pharmacology and drug research. Pregnancy risk factors together with an increased incidence of chronic diseases and a rise in the average maternal age predict medication use will continue to rise during gestation. Common exposure categories include over-the-counter (OTC) medication, psychiatric agents, gastrointestinal medications, herbals, vitamins, antibiotics, and topical products. Only a few medications have been tested specifically for safety and efficacy during human gestation. Profound physiologic changes occur during both normal and pathologic pregnancy that may dramatically alter drug clearance, efficacy, and safety. Under such circumstances, the danger of a drug to mothers, their fetuses, and nursing infants cannot be determined with any confidence until it has been widely used. It is important that women with medical disorders such as diabetes, hypertension, epilepsy, and inflammatory bowel disease continue necessary therapy while pregnant. Unfortunately, many physicians stop or delay medically important agents precisely because of the lack of information.