MEDizzy - A 13-year-old boy suffers two tonic-clonic seizures within 1

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General principles of drug action in pharmacology

A 13-year-old boy suffers two tonic-clonic seizures within 1 week. He is diagnosed with epilepsy, and phenytoin therapy is started. To achieve proper drug concentrations in plasma, the patient is first given a loading dose, followed by maintenance doses. The blood level of phenytoin is frequently monitored to adjust the maintenance dose as needed. What is the rationale behind such a regimen?

ExplanationThe rationale for the loading dose is to give a patient a sufficient dose of a medication to achieve the desired effect quickly, which is necessary in some situation (such as prevention of further seizures). When drug is administered at maintenance rate, steady state is achieved after about five half-lives. The maintenance dose is usually equal to the elimination rate. The loading dose depends on the volume of distribution, whereas the maintenance dose depends on the clearance of the drug.