PharmacologyTier 1Medical Sciences concept

Loading dose calculation (Vd x target concentration / bioavailability)

Core concept

Loading dose = Vd x target concentration / bioavailability (F)
Purpose: rapidly achieve therapeutic concentration without waiting for steady state (~4-5 half-lives)
Depends only on Vd, not on clearance or half-life -- distinct from maintenance dosing (which depends on CL)

IV loading dose: F = 1, so loading dose = Vd x target concentration
Oral loading dose divides by bioavailability -- lower F -> larger oral loading dose needed for the same target level
Larger Vd (lipophilic drugs, oedema, obesity) -> larger loading dose required, independent of renal/hepatic function

Loading dose is not reduced in renal or hepatic impairment -- Vd usually unchanged; only maintenance dose is reduced
- Common trap: reducing the loading dose "to be safe" in renal impairment -- this under-doses and delays therapeutic effect
Used where rapid effect matters: digoxin, phenytoin, amiodarone loading, some antibiotics in sepsis
Toxicity risk from loading dose relates to Vd misestimation (e.g. oedematous patient with a drug that doesn't distribute into oedema fluid) rather than clearance

Contrast with half-life = 0.693 x Vd / CL (see separate note) -- loading dose ignores CL entirely
Maintenance/infusion rate at steady state = CL x target concentration -- the clearance-dependent counterpart
Highly protein-bound, low-Vd drugs (e.g. warfarin) need comparatively small loading doses relative to their potency

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