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USMLE Step 1Pharmacology: principles and autonomic drugsEP 08 of 16

Loading dose and maintenance dose, explained

A drug with a long half-life takes days to reach target on its own. The loading dose fills the volume of distribution; the maintenance dose replaces what clearance removes.

  • 5:02 video
  • 3 min read
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  • Published 15 Nov 2026
Video lesson · 5:02 · Watch on YouTubeSeries: Pharmacology: Pharmacokinetics & Autonomic Drugs · @medservatory
Chapters (10)
By the end of this lesson

Calculate loading and maintenance doses from target concentration, volume of distribution, clearance and bioavailability: loading dose = Css × Vd / F; maintenance dose rate = Css × CL / F.

The short version

  1. Loading dose = target concentration × volume of distribution, divided by F.
  2. Maintenance dose rate = target concentration × clearance, divided by F.
  3. F is the fraction of the dose that reaches the circulation: 1 for an intravenous dose, and both doses are divided by F for other routes.
  4. A steady dose alone takes about five half-lives to reach steady state, so loading is most useful for long half-life drugs such as digoxin.
  5. In organ failure the maintenance dose falls; the loading dose changes only if the volume of distribution or bioavailability changes.

Why use a loading dose and a maintenance dose?

A man with a fast heart rhythm needs a drug today. Its half-life is 36 hours. If you start the usual daily dose, it will be about a week before the level is right.[1][3]

This lesson covers the loading dose and maintenance dose: two calculations that answer two different questions. How do you reach the target concentration now, and how do you stay there?[1]

Volume of distribution, clearance and bioavailability

Two numbers from earlier in the series do all the work. Volume of distribution (Vd) is the size of the container to fill. Clearance (CL) is how fast it drains.[1][2]

A third number, bioavailability (F), is the fraction of the dose that reaches the circulation. It is 1 for an intravenous dose.[1][4][6]

What each dose depends on
DoseQuestion it answersDepends on
Loading doseFill the container: how much drug to reach target now?Target concentration and volume of distribution
Maintenance dose rateReplace what clearance removes: how much to keep it there?Target concentration and clearance

Left to itself, a steady dose takes about five half-lives to reach steady state, which is the reason a loading dose is used at all.[1][3][5]

The loading dose and maintenance dose formulas

Loading dose
Loading dose = Css × Vd / F

Css = target concentration; Vd = volume of distribution; F = bioavailability (1 for IV)

Maintenance dose rate
Maintenance dose rate = Css × CL / F

CL = clearance; the rate is in mass per time, for example mg/h

For any route other than intravenous, divide both by F. A lower F needs a larger dose, because only part of the dose is absorbed.[1][4][6]

At steady state the dose rate equals the rate of elimination, which is clearance multiplied by concentration. To give a dose at fixed intervals, multiply the hourly rate by the interval.[1][7]

Worked example: an intravenous loading dose and maintenance dose

Take a made-up drug: the target concentration is 10 mg/L, the volume of distribution is 40 L and the clearance is 4 L/h. It is given intravenously, so F is 1.[1]

IV example
  1. Loading dose = 10 × 40 = 400 mg
  2. Maintenance dose rate = 10 × 4 = 40 mg/h
  3. Check: clearance removes 4 L of plasma per hour, each litre carries 10 mg, so 40 mg/h is exactly what is lost
  4. Dose every 12 hours: 40 × 12 = 480 mg per dose, which holds the average level at the target

The same drug given orally: F of 0.5

With F of 0.5 both doses double.[1][4]

Oral example
  1. Loading dose = 400 / 0.5 = 800 mg
  2. Maintenance dose rate = 40 / 0.5 = 80 mg/h

Half-life and the five-half-life rule: why bother loading?

In the example the half-life is 0.693 × Vd / CL = 0.693 × 40 / 4, about 7 hours. Maintenance dosing alone would take about 5 × 7 = 35 hours to reach the target. The loading dose gets there at once.[1][3][5]

The loading dose gives the target now; the maintenance dose keeps it there. Loading is most useful for drugs with long half-lives, such as digoxin, which would otherwise take days to reach a therapeutic level.[1][4][5][8]

What changes in organ failure

Kidney or liver failure lowers clearance, so the maintenance dose falls. The loading dose depends on volume and bioavailability, so it usually stays the same.[1]

It changes if the volume has changed: edema enlarges the volume for water-soluble drugs, so a larger loading dose is needed. For an oral drug it also changes if the bioavailability has changed: in liver disease less first-pass metabolism raises F, so a smaller oral loading dose is needed.[1][2]

Two limits are worth knowing. A huge volume can make a loading dose impractically large, and a tiny clearance can make a maintenance dose tiny.[1]

Practice question

Original · USMLE style
Your turnQB-MS-S1-PHARM-012-01

A 58-year-old woman weighing 60 kg needs an oral drug at a target plasma concentration of 15 mg/L. The drug's volume of distribution is 0.35 L/kg, its oral bioavailability is 70 percent and its clearance is 3 L/h.

Which of the following is the most appropriate oral loading dose?
Pick one option. Your answers stay on this device.

Common questions

What is the loading dose formula?

Loading dose = target concentration × volume of distribution, divided by bioavailability (F). For an intravenous dose F is 1.

What is the maintenance dose formula?

Maintenance dose rate = target concentration × clearance, divided by F. Multiply the hourly rate by the dosing interval to get the dose per interval.

Why is a loading dose divided by bioavailability?

Only the fraction F of an oral dose reaches the circulation, so the dose must be larger to deliver the same amount. With F of 0.5, both doses double.

When is a loading dose most useful?

For drugs with long half-lives, such as digoxin. A steady dose alone takes about five half-lives to reach steady state, while a loading dose reaches the target at once.

Does the loading dose change in kidney or liver failure?

Usually not. Clearance falls, so the maintenance dose falls, but the loading dose changes only if the volume of distribution or bioavailability changes.

Transcript

Read the full narration, by chapter632 words

0:00The case: a 36-hour half-life and a drug needed today

A man with a fast heart rhythm needs a drug today. Its half-life is 36 hours. Start the usual daily dose, and it will be about a week before the level is right. So how do you reach the target today, and then stay there? Today: calculate loading and maintenance doses from target concentration, volume of distribution, clearance and bioavailability.

0:23Two numbers do the work: Vd, clearance, F and five half-lives

Two numbers from earlier in this series do all the work. Volume of distribution is the size of the container to fill. Clearance is how fast it drains. Bioavailability, called F, is the fraction of the dose that reaches the circulation. It is one for an intravenous dose. And left to itself, a steady dose takes about five half-lives to reach steady state.

0:48The two formulas: loading dose and maintenance dose rate

The loading dose fills the container: target concentration times volume of distribution. The maintenance dose rate replaces what clearance removes: target concentration times clearance. For any route other than intravenous, divide both by F.

1:05Worked example (IV): 400 mg to load, 40 mg/h to maintain

A worked example. The target concentration is 10 milligrams per litre, the volume is 40 litres, and the clearance is four litres per hour. The drug is given intravenously. The loading dose is 10 times 40: 400 milligrams. The maintenance dose rate is 10 times four: 40 milligrams per hour. That makes sense. Clearance clears four litres of plasma per hour, and each litre carries 10 milligrams, so 40 milligrams per hour is exactly what is lost. For a dose every 12 hours, multiply the hourly rate by 12: 480 milligrams per dose, which holds the average level at the target.

1:49The same drug orally: F of 0.5 doubles both doses

Now give the same drug orally, with F of 0.5. Both doses double: 800 milligrams to load, and 80 milligrams per hour to maintain.

2:00Why bother loading? Half-life and the five-half-life rule

Why bother loading? The half-life here is 0.693 times 40 over four, about 7 hours. Maintenance dosing alone would take about 35 hours to reach the target. The loading dose gets there at once. The loading dose gives the target now. The maintenance dose keeps it there. Loading is most useful for drugs with long half-lives, such as digoxin. Without it they would take days to reach a therapeutic level.

2:31What changes in organ failure

Which dose changes in organ failure? Kidney or liver failure lowers clearance, so the maintenance dose falls. The loading dose depends on volume and bioavailability, so it usually stays the same, unless the volume has changed, as with edema, or, for an oral drug, the bioavailability has changed, as in liver disease. A huge volume can make a loading dose impractically large, and a tiny clearance can make a maintenance dose tiny.

3:02Practice question: an oral loading dose

Your turn. A 58-year-old woman weighing 60 kilograms needs an oral drug at a target plasma concentration of 15 milligrams per litre. The drug's volume of distribution is 0.35 litres per kilogram, its oral bioavailability is 70%, and its clearance is three litres per hour. Which of the following is the most appropriate oral loading dose? A, 64 milligrams. B, 220 milligrams. C, 315 milligrams. D, 450 milligrams. E, 640 milligrams. Pause here and decide.

3:47Answer and explanation

A is out: that is the maintenance dose rate, target times clearance over F, in milligrams per hour. B is out: it multiplies by bioavailability instead of dividing, so it gives too little drug. C is out: 315 ignores bioavailability, so only 70% of it would be absorbed. E is out: it divides by bioavailability twice. So the answer is D. The volume is 0.35 times 60, 21 litres. Fifteen times 21 is 315 milligrams, and dividing by 0.7 gives 450.

4:29Recap and next lesson

Three things to keep. One: loading dose equals target concentration times volume of distribution, divided by F. Two: maintenance dose rate equals target concentration times clearance, divided by F. Three: in organ failure, cut the maintenance dose; the loading dose changes only if volume or bioavailability changes. Next, CYP450 inducers and inhibitors.

Sources

Numbers in the text, e.g. [1], point here
  1. Deranged Physiology. Maintenance dose and loading dose. derangedphysiology.com/main/cicm-primary-exam/pharmacokinetics/Chapter-401/maintenance-dose-and-loading-dose
  2. Deranged Physiology. Volume of distribution. derangedphysiology.com/main/cicm-primary-exam/pharmacokinetics/Chapter-202/volume-distribution
  3. Deranged Physiology. Half-life. derangedphysiology.com/main/cicm-primary-exam/pharmacokinetics/Chapter-322/half-life
  4. Wikipedia. Loading dose. en.wikipedia.org/wiki/Loading_dose
  5. Wikipedia. Biological half-life. en.wikipedia.org/wiki/Biological_half-life
  6. MSD Manual Professional Version. Overview of Pharmacokinetics. msdmanuals.com/professional/clinical-pharmacology/pharmacokinetics/overview-of-pharmacokinetics
  7. Benet LZ, Sodhi JK. AAPS J (Europe PMC PMC13171289). europepmc.org/article/PMC/PMC13171289
  8. DailyMed. Lanoxin (digoxin) tablets, US prescribing information. dailymed.nlm.nih.gov/dailymed/services/v2/spls/d91e3646-4c63-4512-ab22-db39c085c4dc.xml

The patient, the drug and every dose on screen are teaching values written for this lesson. Worked example: 10 x 40 = 400 mg; 10 x 4 = 40 mg/h; 40 x 12 = 480 mg; 400 / 0.5 = 800 mg; 40 / 0.5 = 80 mg/h; half-life 0.693 x 40 / 4 = about 7 h; five half-lives = about 35 h.

The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.

Image credits. Illustrations: Servier Medical Art (smart.servier.com), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (recoloured, outlined, resized): lying man, stomach, syringe, kidney, liver. Calendar and infusion pump icons: Health Icons (healthicons.org), CC0. Pill icon by Marcel Tisch via Bioicons (bioicons.com), CC0. Calculator icon by OpenClipart via Bioicons, CC0. The tank, graphs, dots, formulas, cards and all handwriting are drawn in code (original). The heart-rate trace is a schematic. No AI-generated teaching images.

Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-PHARM-012.

For exam study only, not medical advice. Read the medical disclaimer.

In this series

Playlist on YouTube
← Previous · EP 07Zero-order vs first-order elimination explained
  1. EP 01Potency vs efficacy: EC50, Emax and agonist types5:17
  2. EP 02Competitive vs noncompetitive antagonists: effect on EC50 and Emax5:01
  3. EP 03Therapeutic index explained: TD50 / ED505:01
  4. EP 04First-pass metabolism and bioavailability: calculating oral F from AUC5:18
  5. EP 05Volume of distribution: why Vd can be bigger than the body5:17
  6. EP 06Clearance and half-life: t½ = 0.693 × Vd / CL5:01
  7. EP 07Zero-order vs first-order elimination explained5:12
  8. EP 08Loading dose and maintenance dose, explained5:02
  9. EP 09CYP450 inducers and inhibitors: how they change drug levels5:16
  10. EP 10Autonomic receptors: alpha, beta and muscarinic5:33
  11. EP 11Beta blocker selectivity: beta-1, ISA and alpha blockade5:24
  12. EP 12Vasopressors compared: receptors, pressure and heart rate5:46
  13. EP 13Organophosphate poisoning: muscarinic vs nicotinic signs5:36
  14. EP 14Receptor desensitization, downregulation and upregulation5:01
  15. EP 15Beta-lactams and vancomycin: enzyme versus substrate5:04
  16. EP 16Protein synthesis inhibitors: 30S vs 50S antibiotics5:07