Dose Regimens Dose Regimens a dose regimen is a plan for drug administration over a period of time an optimal dose regimen results in achievement of therapeutic levels of a drug in blood without the exceeding minimum toxic concentration if it is necessary to achieve target plasma levels of a drug rapidly, a loading dose is used to "load" Vd with drug to maintain plasma concentration of drug within a specified range over long periods of therapy, a schedule of maintenance doses is used Definitions Cp = target plasma concentration F = bioavailability F = 1 with IV administration adjusted body weight dosing used for lipophilic drugs in obese individuals Loading Dose Loading dose = Cp x Vd/F loading dose is dose required to achieve a specific plasma drug concentration level with a single administration loading dose will change depending on the volume of distribution in the patient if loading dose is very large (Vd much larger than blood volume), dose should be given slowly to prevent toxicity due to excessively high plasma levels during distribution phase in a patient with impaired kidney/liver function, loading dose remains unchanged Maintenance Dose Maintenance dose = Cp x CL/F maintenance dose is dose required to maintain a target plasma drug concentration level in a patient with impaired kidney/liver function, maintenance dose decreases post-antibiotic effect: once plasma drug concentration falls, there is still antibiotic effects that suppress bacterial growth