A loading dose is one dose or a series of doses given at the onset of therapy with the aim of achieving the target concentration rapidly. The appropriate magnitude for the loading dose is:
A loading dose may be desirable if the time required to attain steady state (and efficacy) by the administration of drug at a constant rate (four half-lives) is long relative to the demands of the condition being treated as is the case with the treatment of arrhythmias or cardiac failure.
The use of a loading dose also has significant disadvantages. The patient may be exposed abruptly to a toxic concentration of a drug that may take a long time to fall (i.e., long ty2). Loading doses tend to be large, and they are often given parenterally and rapidly; this can be particularly dangerous if toxic effects occur as a result of actions of the drug at sites that are in rapid equilibrium with the high concentration in plasma. It is therefore usually advisable to divide the loading dose into a number of smaller fractional doses administered over time, or to administer the loading dose as a continuous intravenous infusion over a period of time using computerized infusion pumps.
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