PHC5005 Chap.5 Half-Life, Steady State and Dosing Regimens
Half-Life, Steady State and Dosing Regimens
From parameters to a regimen
The first four chapters describe how the body handles a drug. This one turns those descriptions into dosing decisions.
The course's tutorial frames it through four primary parameters, bioavailability, rate of absorption, volume of distribution and clearance, and asks how each shapes the derived quantities a prescriber actually works with: half-life, time to steady state, the steady-state concentration, the swing between peak and trough, and accumulation.
Half-life
The half-life is the time needed for the plasma concentration to fall by half.
It is proportional to volume of distribution and inversely proportional to clearance, so tissue sequestration lengthens it and efficient elimination shortens it. One half-life removes 50 per cent of the drug, two remove 75 per cent, three 87.5 per cent and four 93.75 per cent, which is why nearly complete elimination takes four to five half-lives.
Bioavailability, by contrast, changes how much drug arrives but not the half-life or the time to steady state.
Steady state and the plateau principle
When a constant dose is repeated before the previous dose is gone, levels climb until elimination balances input.
At that point, steady state, the rate in equals the rate out and concentrations fluctuate around an average equal to the dose rate divided by clearance. Reaching it takes four to five half-lives regardless of dose. The tutorial measures two features of the plateau: fluctuation, the steady-state peak divided by the trough, and accumulation, the steady-state average divided by the first-dose average.
Dosing more often than the half-life favours accumulation; dosing less often favours fluctuation.
Loading and maintenance doses
Short half-life drugs such as amoxicillin and erythromycin are dosed at conventional intervals and the fluctuation is accepted.
Long half-life drugs would take too long to reach steady state, so they start with a loading dose, the target concentration multiplied by the volume of distribution, and continue with a maintenance dose that replaces what is cleared.
The lecture warns that loading doses are large and can expose the patient abruptly to high levels, and suggests dividing them or giving them by infusion.
The therapeutic window and monitoring
The aim of any regimen is a steady-state level inside the therapeutic window, above the minimum effective concentration and below toxicity.
Narrow windows leave no room for variability, and the tutorial's simulated population of two hundred patients on the same antibiotic dose shows how much variability there is. Therapeutic drug monitoring, measuring plasma levels, is indicated for narrow therapeutic index drugs, potentially toxic drugs and poisoning.
What this chapter covers
- 01
Half-life as a derived parameter of volume and clearance
- 02
Elimination percentages after one to four half-lives
- 03
Steady state, the plateau principle and time to plateau
- 04
Fluctuation and accumulation as measured in the tutorial
- 05
Loading dose, maintenance dose and the risks of loading
- 06
Parameter changes from dehydration, organ failure and dialysis
- 07
Therapeutic window, population variability and drug monitoring
Worked example · free
Predicting time to steady state and the effect of a dose change
- 2Estimate the time to steady state from the half-life.
- 2State the effect of doubling the dose rate on the average level.
- 1State the effect on the time to reach it.
Key terms
- Half-Life
- How long it takes the plasma concentration of a drug to halve.
- Steady State
- The condition in which the rate of drug administration equals the rate of elimination, so levels plateau.
- Plateau Principle
- The rise of repeated-dose levels to a plateau when each dose is given before the previous one is eliminated.
- Fluctuation
- The steady-state peak concentration divided by the trough that follows it.
- Accumulation
- The steady-state average concentration divided by the average after the first dose.
- Maintenance Dose
- The dose repeated at fixed intervals after loading to keep the steady-state concentration by replacing what is eliminated.
- Therapeutic Window
- The plasma concentration range that gives efficacy without unacceptable toxicity.
- Therapeutic Drug Monitoring
- Measurement of plasma drug concentrations to ensure safety and efficacy, used for narrow-index and toxic drugs.
Half-Life, Steady State and Dosing Regimens FAQ
Does a bigger dose reach steady state faster?
No. The time to steady state depends on the half-life alone, four to five half-lives whatever the dose. A larger dose rate raises the height of the plateau in proportion but not the speed of approach; a loading dose is the way to reach the target sooner.
Why does it take four to five half-lives to eliminate a drug?
Each half-life removes half of what remains, so the fractions eliminated run 50, 75, 87.5 and 93.75 per cent. After four to five half-lives only a few per cent is left, which the course treats as practically complete elimination.
What is the difference between fluctuation and accumulation?
Fluctuation compares the peak and trough within one interval at steady state. Accumulation compares the steady-state average with the average after the very first dose. Frequent dosing relative to the half-life increases accumulation, infrequent dosing increases fluctuation.
Why can a loading dose be dangerous?
It is usually large and exposes the patient suddenly to a high concentration. If toxicity appears and the half-life is long, the level falls slowly. The course suggests dividing the loading dose into smaller fractions over a period or giving it as a controlled intravenous infusion.
How does dehydration change drug levels?
Losing body water shrinks the volume of distribution. The same dose then produces a higher peak concentration, and because half-life is proportional to volume, the half-life shortens. The tutorial pairs this with fluid overload in organ failure, which does the opposite.
When is therapeutic drug monitoring used?
The course gives three indications: drugs with a narrow therapeutic index or low safety margin, potentially toxic drugs, and poisoning or overdose. Its examples include warfarin, digoxin, lithium, phenytoin, vancomycin, gentamicin, tacrolimus and paracetamol.
Exam move
Draw two axes and sketch, without notes, a single dose decaying, repeated doses climbing to a plateau, and a loaded regimen jumping straight to it. Label the half-life, the plateau, the peak and the trough on each. If you can draw them, every question about time to steady state, accumulation or loading reduces to reading your own sketch.
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