If terrorists released Bacillus anthracis over a large city, hundreds of thousands of people could be at risk of the deadly disease anthrax-caused by the B. anthracis spores-unless they had rapid access to antibiotic medical countermeasures (MCM). Although plans for rapidly delivering MCM to a large number of people following an anthrax attack have been greatly enhanced during the last decade, many public health authorities and policy experts fear that the nation's current systems and plans are insufficient to respond to the most challenging scenarios, such as a very large-scale anthrax attack. The U.S. Department of Health and Human Services' Office of the Assistant Secretary for Preparedness and Response commissioned the Institute of Medicine to examine the potential uses, benefits, and disadvantages of strategies for repositioning antibiotics. This involves storing antibiotics close to or in the possession of the people who would need rapid access to them should an attack occur. Prepositioning Antibiotics for Anthrax reviews the scientific evidence on the time window in which antibiotics successfully prevent anthrax and the implications for decision making about prepositioning, describes potential prepositioning strategies, and develops a framework to assist state, local, and tribal public health authorities in determining whether prepositioning strategies would be beneficial for their communities. However, based on an analysis of the likely health benefits, health risks, and relative costs of the different prepositioning strategies, the book also develops findings and recommendations to provide jurisdictions with some practical insights as to the circumstances in which different prepositioning strategies may be beneficial. Finally, the book identifies federal- and national-level actions that would facilitate the evaluation and development of prepositioning strategies. Recognizing that communities across the nation have differing needs and capabilities, the findings presented in this report are intended to assist public health officials in considering the benefits, costs, and trade-offs involved in developing alternative prepositioning strategies appropriate to their particular communities.