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Loading dose: reaching target levels faster
Medically reviewed by Javeria Muneer, Pharmacist · Last reviewed:
A loading dose is a larger first dose that brings plasma levels close to the target straight away, instead of waiting four to five half-lives for regular doses to build up. It is used when a delay would be harmful or when the half-life is long, for example with digoxin, amiodarone, vancomycin in serious infection or some antiepileptics.
The calculation
Loading dose = Target concentration × V / F. Because it fills the volume of distribution, it depends on V, not on clearance. A patient with a larger volume, such as someone with fluid overload, may need a bigger loading dose even if their kidneys are fine.
The maintenance dose, by contrast, replaces what is eliminated each interval and depends on clearance: Maintenance dose = Target average concentration × CL × τ / F.
Practical points
Loading doses carry a higher risk of peak-related side effects, so they are sometimes split or given slowly. Kidney impairment usually changes the maintenance dose but not the loading dose.
See it in the simulator
Compare a regimen with a doubled first dose against regular dosing using the Compare feature on the full simulator.
