Once a diagnosis has been made and medical treatment is deemed necessary, safe and effective pharmacologic agents that exert the appropriate actions should be selected. A dosing regimen should be individualized for each patient. In addition to the route, which often is based on drug availability or convenience, a number of factors should be considered when designing a dosing regimen. These include host considerations that may alter the response to or disposition of the drugs. Adjustment in route, dose, or interval may be indicated based on host and drug factors. For antimicrobial drugs, microbial factors, including resistance, also should be considered. Finally, particularly for food animals, public health, environmental implications, and regulatory constraints must be considered.