Maintenance Dose
Recurring dose needed to sustain the steady-state concentration.
Formula
Variables
Example
Css = 10 mg/L, CL = 6 L/hr, τ = 12 hr → MD = 720 mg
How it works
This formula estimates the amount of drug that must be given at each dosing time so the body replaces exactly what is eliminated during one dosing interval. It comes from the idea that, at steady state, the average amount removed by clearance over time should match the amount administered over that same period. Multiplying clearance by the target steady-state concentration gives the elimination rate, and multiplying by the dosing interval converts that rate into a per-dose amount.
When to use it
Use this when planning repeated dosing for a drug given at regular intervals and you want to maintain a target blood concentration. It is especially useful for IV infusions converted to intermittent dosing or for oral/IV maintenance regimens in pharmacokinetics.
Worked example, step by step
Given: Css = 8 mg/L, CL = 4.5 L/hr, τ = 8 hr Step 1: Write the formula: MD = Css × CL × τ Step 2: Substitute values: MD = 8 mg/L × 4.5 L/hr × 8 hr Step 3: Multiply units: (mg/L)(L/hr)(hr) = mg Step 4: Calculate: MD = 8 × 4.5 × 8 = 288 mg Final answer: The maintenance dose is 288 mg per dose.
Common mistakes to avoid
- Using the loading dose formula instead of the maintenance dose formula
- Forgetting to include the dosing interval, which changes a rate into an amount per dose
- Mixing up units for clearance and concentration so the final dose has inconsistent units
Did You Know?
If the maintenance dose is too low, the drug dips below the therapeutic window between doses — the 'trough' effect.
Share this formula
More in Pharmacology & Dosage
View allDrug Half-Life
IntermediateTime for the plasma concentration of a drug to fall to half its initial value.
Steady-State Concentration
IntermediateAverage plasma drug concentration when intake and elimination are balanced.
Loading Dose
IntermediateInitial higher dose to rapidly achieve the desired plasma concentration.
Volume of Distribution
BasicApparent volume into which a drug distributes — a larger Vd means the drug leaves the bloodstream and enters tissues.
Clearance
IntermediateVolume of plasma completely cleared of a drug per unit time.
Bioavailability
IntermediateFraction of an oral dose that reaches systemic circulation compared with an IV dose.