Extracts of milk thistle have been recognized for centuries as "liver tonics" and are well-known to prevent or reverse hepatotoxicity of reactive drug metabolites or naturally occurring toxins. Milk thistle extracts are now under intense study in the experimental therapeutics of cancer for chemoprevention, treatment, and amelioration of chemotherapy side effects. Precision in nomenclature, however, has lagged behind this progress. The crude commercial product of milk thistle is termed silymarin, a complex of at least 7 flavonolignans and 1 flavonoid that comprises 65% to 80% of milk thistle extract. From silymarin is derived silibinin, a semipurified fraction once thought to be a single compound but now recognized as a 1:1 mixture of 2 diastereoisomers, silybin A and silybin B. The distinction between silymarin and silibinin is not only important to understanding the historical literature, but thorough characterization and use of chemically defined mixtures in preclinical and clinical studies are essential to the progress of these botanical compounds as human therapeutics. As a result, we urge clinicians and preclinical investigators alike to exercise rigor in nomenclature and use pure compounds or precisely defined chemical mixtures in subsequent studies. Herein, we provide a guide to the proper nomenclature and composition of milk thistle extracts and discuss the known pharmacokinetic studies of these botanical medicines. The drug-interaction potential of these extracts appears to be quite low, and in fact, silibinin appears to synergize with the antitumor effects of some commonly used chemotherapeutics. However, some precautions are advised as high-dose, phase II studies are conducted.