loading dose
A loading dose is a large opening dose given to fill the body up to the therapeutic level fast, instead of waiting the usual 4–5 half-lives it takes to reach steady state by ordinary dosing. Think of priming a pump or filling a bathtub quickly before settling into a slow trickle to keep it topped up — the loading dose is that initial gush.
Its size is set by how much room there is to fill, not by the rate of elimination: loading dose ≈ target concentration × volume of distribution. Because a large volume of distribution means a big tank to fill, drugs that spread widely into tissues often need substantial loading doses. Clearance does not enter the calculation here.
Loading doses are valuable when a delay would be harmful — a life-threatening arrhythmia, a serious infection, status epilepticus. The trade-off is risk: delivering a big dose all at once can briefly overshoot into toxic concentrations, so loading doses are often split or infused slowly, and may be unwise for drugs with a narrow therapeutic window.
Intravenous digoxin or amiodarone is started with a loading dose because their long half-lives would otherwise mean days or weeks before reaching an effective level.
Long-half-life drugs need loading to act quickly.
Loading dose is set by volume of distribution; maintenance dose is set by clearance. They answer different questions and use different parameters.