A rational dosage regimen considers a drug's pharmacokinetics, including its absorption, distribution, metabolism, and elimination from the body. By understanding these factors, the appropriate dosage can be determined, and the dosing schedule can be designed to achieve and maintain the desired therapeutic effect while minimizing adverse effects.

In most cases, drugs are administered repetitively or infused continuously to maintain a steady-state concentration in the body. At a steady state, the rate of drug administration is balanced by the rate of drug elimination. This allows consistent and effective drug concentration to be maintained over time.

The maintenance dose refers to the quantity of medication required to maintain a therapeutic concentration in the body over an extended period. It is often adjusted based on factors such as the patient's age, weight, renal function, and desired therapeutic outcomes. The maintenance dose ensures that the drug remains in a steady state within the body, balancing the elimination rate and the rate of drug administration.

Waiting for a drug to attain a steady state may take too long for immediate and urgent situations that require a rapid response. In such cases, a high drug concentration called loading dose is often administered initially to achieve the desired therapeutic levels quickly. The loading dose is calculated based on the drug's volume of distribution (V_d), target plasma concentration, and bioavailability. The volume of distribution represents the apparent space in the body where the drug is distributed.

Once the target therapeutic levels are reached with the loading dose, maintenance dosing is followed to sustain the desired drug concentration over time. This approach allows for a rapid onset of action while ensuring that the drug remains at adequate levels for the necessary duration of treatment.