Steady-State Concentration
Average plasma drug concentration when intake and elimination are balanced.
Formula
Variables
Example
F=1, D=500 mg, CL=5 L/hr, τ=8 hr → Css = 12.5 mg/L
How it works
This formula estimates the average drug concentration in plasma after repeated dosing has reached steady state, meaning the amount entering the body over each dosing interval matches the amount removed. It works because, at equilibrium, the total drug delivered per interval is effectively divided by how efficiently the body clears the drug. Higher bioavailability or dose raises the concentration, while higher clearance or a longer time between doses lowers it.
When to use it
Use this when you need a quick estimate of the average steady-state level for a drug given repeatedly at regular intervals, such as in maintenance dosing or therapeutic drug monitoring. It is especially helpful for comparing how changes in dose, interval, bioavailability, or clearance affect expected exposure.
Worked example, step by step
Given: F = 0.75, D = 200 mg, CL = 10 L/hr, τ = 12 hr Step 1: Multiply bioavailability by dose: F × D = 0.75 × 200 = 150 mg Step 2: Multiply clearance by dosing interval: CL × τ = 10 × 12 = 120 L Step 3: Divide: Css = 150 / 120 = 1.25 mg/L Final answer: Css = 1.25 mg/L
Common mistakes to avoid
- Using the total daily dose without matching it to the dosing interval in the formula
- Forgetting that bioavailability is a fraction between 0 and 1, not a percentage unless converted
- Mixing units so that clearance and dose interval do not produce concentration units
Did You Know?
Steady state is typically reached after about five half-lives of repeated dosing.
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More in Pharmacology & Dosage
View allDrug Half-Life
IntermediateTime for the plasma concentration of a drug to fall to half its initial value.
Loading Dose
IntermediateInitial higher dose to rapidly achieve the desired plasma concentration.
Maintenance Dose
IntermediateRecurring dose needed to sustain the steady-state 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.