3.27: Rational Dosage Regimen: Maintenance Dose and Loading Dose

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.

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 V_d, target plasma concentration and bioavailability. Upon reaching the required therapeutic levels,  maintenance dosing is followed to sustain the target concentration.