One of the least-developed areas of clinical pharmacology and drug research is the use of medication during pregnancy and lactation. This article is the first in a two-part series designed to familiarize physicians with many aspects of the drugs they commonly prescribe for pregnant and breast-feeding women. Almost every pregnant woman is exposed to some type of medication during pregnancy. Although the majority of pregnant and breast-feeding women consume clinically indicated or over-the-counter drug preparation regularly, only few medications have specifically been tested for safety and efficacy during pregnancy. There is scant information on the effect of common pregnancy complications on drug clearance and efficacy. Often, the safety of a drug for mothers, their fetuses, and nursing infants cannot be determined until it has been widely used. Absent this crucial information, many women are either refused medically important agents or experience potentially harmful delays in receiving drug treatment. Conversely, many drugs deemed "safe" are prescribed despite evidence of possible teratogenicity. Novel research and diagnostic applications evolving from the opportunities presented by the advances in genomics and proteomics are now beginning to affect clinical diagnosis, vaccine development, drug discovery, and unique therapies in a modern diagnostic-therapeutic framework-part of the new scientific field of theranostics. This review critically explores a number of recently raised issues in regard to the use of several classes of medications during gestation and seeks to provide a general and concise resource on drugs commonly used during pregnancy and lactation. It also seeks to make clinicians more aware of the controversies surrounding some drugs in an effort to encourage safer prescribing practices through consultation with a maternal-fetal medicine specialist and through references and Web sites that list up-to-date information.