A rational dosage regimen describes the specific plan to administer a drug to establish a target drug concentration over time. Doing so produces the desired therapeutic results with minimal adverse effects. The target concentration depends on the drug's absorption, distribution, and elimination rates. Usually, drugs are administered repetitively or infused continuously to maintain a steady-state of drug concentration, where the rates of administration and elimination are equal. Such maintenance dosing rate depends on the target plasma concentration Cp, drug clearance CL, and the bioavailability of the drug, F. For serious conditions such as an infection that needs an immediate response, the usual time taken to establish steady-state may be too long. So, the treatment starts with a large loading dose to quickly reach the required therapeutic levels and generate a rapid effect. The loading dose depends on the drug's volume of distribution Vd, target plasma concentration and bioavailability. Upon reaching the required therapeutic levels, maintenance dosing is followed to sustain the target concentration.