The drugs used in clinical oncology are inherently toxic because they are designed to kill host cells. For the most part, the toxic effects occur in tissues with high growth fractions--that is, bone marrow, gastrointestinal epithelium, and hair follicles. Nevertheless, certain drugs do cause harm to other organs. At therapeutic doses of anticancer drugs, the toxicoses are usually mild to moderate and can be managed without undue discomfort to the animal. Occasionally, however, life-threatening cytopenias occur. By contrast, with drug overdose, life-threatening toxicoses can be a very real concern.