loading dose
A loading dose is a larger-than-usual first dose given to get a drug working fast. Without it, a drug taken at its normal repeating dose creeps up to its effective level only gradually, over several half-lives. The loading dose is a shortcut: a single big hit that fills the body to the target concentration immediately, after which smaller maintenance doses simply hold it there.
The size of a loading dose is set not by clearance but by the volume of distribution — the apparent space the drug must fill. To reach a target concentration, you give roughly the volume of distribution times that concentration, so a drug that spreads widely into the tissues needs a proportionally bigger loading dose to saturate them.
Loading doses are most useful when waiting days for a drug to accumulate is unacceptable, as with a serious infection, a dangerous heart rhythm, or a seizure. The trade-off is that a big up-front dose can momentarily push levels high, so for drugs with a narrow safety margin the loading dose must be calculated carefully or split to avoid overshooting into toxicity.
An antibiotic with a long half-life would take a day or more to reach therapeutic levels on maintenance dosing alone, so a doubled first dose is given to hit the target concentration in a serious infection right away.
A loading dose buys time you cannot afford to wait.
A loading dose scales with the volume of distribution (loading dose ≈ Vd × target concentration), whereas the maintenance dose scales with clearance — two different parameters governing two different decisions.