# Loading and Maintenance Dose > Loading and maintenance dose in MBBS Pharmacology: loading = Vd × target Cp formula, maintenance = CL × Css, digoxin and amiodarone worked examples. - Canonical URL: https://prepelephant.com/topics/mbbs/pharmacology/loading-maintenance-dose - Exam / course: MBBS · Subject: Pharmacology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Loading and Maintenance Dose", PrepElephant, https://prepelephant.com/topics/mbbs/pharmacology/loading-maintenance-dose ## Direct answer A loading dose exists to fill the body's volume of distribution quickly — loading dose = Vd × target plasma concentration / bioavailability — so that effective levels appear immediately instead of after five half-lives of accumulation; it depends on volume, never on clearance. The maintenance dose replaces what elimination removes each day — maintenance dose = CL × target Css × dosing interval / bioavailability — and depends on clearance, never on volume. Amiodarone (half-life of weeks) loads with grams over days, digoxin loads with divided microgram doses, and phenytoin status epilepticus loads at 20 mg/kg precisely because waiting for steady state would mean waiting while seizures continue. When the two are confused, patients are either underdosed for days or poisoned on day one. ## What you must remember - **Loading formula:** loading dose = Vd × target Cp / F; a 70 kg man needing phenytoin 20 mg/L with Vd 0.65 L/kg loads at 20 × 45 ≈ 900 mg — the familiar "1 g" of status protocols (rate not exceeding 50 mg/min, cardiac monitoring). - **Maintenance formula:** dose per dosing interval = CL × Css × τ / F; when clearance halves, halve the dose or double the interval — the entire logic of renal dose adjustment. - **Why load:** at constant maintenance dosing, only 50 percent of steady state exists after one half-life; for digoxin (t½ about 40 hours) that means days of underdosing — a loading dose buys time. - **Amiodarone logic:** enormous Vd (thousands of litres) and a half-life of one to two months; oral loading 200 mg thrice daily for a week or two, tapering to 200 mg daily — without loading, therapeutic levels take a month. - **Digoxin loading:** total about 0.75-1 mg orally in divided doses (for example 0.5, 0.25, 0.25 mg six hours apart) for af with rapid ventricular rate; halve in renal impairment, elderly, or hypokalaemia-prone patients. - **When NOT to load:** narrow therapeutic index drugs where volume is unstable, drugs whose target concentration is uncertain, and patients at high toxicity risk — or drugs with short half-lives and modest Vd, where steady state arrives within hours anyway. - **Bioavailability discipline:** oral loading must be divided by F, and IV loading demands slower rates (phenytoin, vancomycin over 60 minutes) to avoid rate-related toxicity. - **Accumination check:** after any loading-plus-maintenance start, levels confirm the arithmetic — phenytoin and vancomycin loading levels are checked after distribution completes, not at the peak of infusion. ## A ward round with both formulas A 62-year-old is admitted with atrial fibrillation at 160 per minute and heart failure. The plan: digoxin loading 0.5 mg orally, then 0.25 mg after 6 hours, then a final 0.25 mg — deliberately short of the full calculated load because his creatinine of 1.8 mg/dL predicts a low maintenance need. Maintenance starts at 0.125 mg next day, a clearance-driven decision; potassium is checked because hypokalaemia potentiates toxicity at any level. Contrast the bed opposite: a woman starting amiodarone for paroxysmal af gets 200 mg thrice daily for a week, then twice daily for a week, then 200 mg daily — the stepped oral load that compresses a month of accumulation into days, then maintenance at the smallest dose holding sinus rhythm, with thyroid and liver tests at baseline. The third bed, status epilepticus, gets phenytoin 20 mg/kg intravenously — the purest loading-dose moment in medicine, existing because seizures cannot wait for pharmacokinetic equilibrium. ## Where students slip The classic viva error is assigning clearance a role in loading: the examiner asks "the patient's renal function has fallen — does the loading dose change?" and the candidate wrongly halves it. Volume and target set loading; renal function sets maintenance. The second error is loading to toxic targets — copying amiodarone logic onto digoxin arithmetic without dividing by F or considering renal impairment, producing vomiting and heart block. The third is rate-blindness: phenytoin 1 g is safe over 20 minutes and dangerous over 2 — hypotension and bradycardia are infusion-rate effects, and the purple-glove risk rises with small veins and fast pushes. A final favourite: gentamicin's large extended-interval dosing follows peak-kill and post-antibiotic logic, not accumulation logic — the exception that proves the rule. ## Frequently asked questions ### What formula gives the loading dose? Loading dose = Vd × target plasma concentration / bioavailability — volume of distribution, not clearance, determines it. ### What formula gives the maintenance dose? Maintenance dose = clearance × target steady-state concentration × dosing interval / bioavailability. ### Why does amiodarone need a loading regimen? Its huge volume of distribution and one-to-two-month half-life mean unaided steady state would take weeks; loading fills tissue compartments within days. ### How is digoxin loaded in practice? About 0.75-1 mg orally in divided doses 6 hours apart (reduced in renal impairment and the elderly), then a clearance-based daily maintenance dose. ### Does renal impairment change the loading dose? Not directly — loading depends on Vd; renal impairment lowers clearance and therefore the maintenance dose, and by extension the safe target.