Steady State

Also known as: steady-state concentration · Css

Steady state is the point in repeated dosing where the drug eliminated each interval equals the drug taken in, so levels stop climbing.

When you dose on a fixed schedule, each new dose adds to what is left of the previous ones, so levels climb at first. As concentration rises, the amount eliminated per interval rises with it, until elimination matches input. From then on, peaks and troughs repeat in the same pattern every interval: that is steady state. It does not mean the level is flat; it means the cycle has stopped changing.

In linear pharmacokinetics, how long it takes depends on the half-life, not on the dose. In a simple model the fraction of steady state reached after n half-lives is 1 − 0.5^n: 50% after one, 75% after two, 87.5% after three, 93.75% after four and about 97% after five. That is where the 4-to-5-half-life rule of thumb comes from. Half-life also sets how much the drug accumulates and how much levels fluctuate once you get there.

Why it matters

Labels for approved peptides line up with that arithmetic. Semaglutide, with a half-life of about 1 week, reaches steady-state exposure after 4 to 5 weeks of once-weekly dosing. Tirzepatide, with a half-life of about 5 days, reaches it after 4 weeks. The same clock restarts every time the dose changes: after an increase, levels take another 4 to 5 half-lives to settle at the new steady state.

This is why early impressions can mislead. In the first weeks of a long-half-life compound, levels are still rising, so what you notice in week one does not reflect exposure at steady state. For a compound with a half-life of a few hours, the opposite holds: the arithmetic puts steady state within about a day (5 × 4 hours = 20 hours), and continuing longer does not push levels higher.

How to calculate it

Find a sourced half-life for the compound.
Multiply it by 4 to 5 to estimate the time to steady state.
For the fraction reached at a given time, count the half-lives elapsed (n) and compute 1 − 0.5^n.
Restart the count from the day of any dose change.

Fraction of steady state after n half-lives = 1 − 0.5^n

Worked example

Tirzepatide, half-life about 5 days: 4 weeks = 28 days = 28 / 5 = 5.6 half-lives. 0.5^5.6 ≈ 0.021, so 1 − 0.021 ≈ 0.979, about 98% of steady state, consistent with the label's 4 weeks. Semaglutide, half-life about 7 days: 4 weeks is 4 half-lives, 1 − 0.0625 = 0.9375 (93.75%); 5 weeks is 1 − 0.03125 ≈ 0.969 (about 97%).

Common mistakes

Judging a weekly compound with a one-week half-life after one or two doses, when levels are only at about 50% to 75% of where they will settle.
Thinking that raising the regular dose shortens the time to steady state; it changes the level, not the time.
Forgetting that each dose change restarts the 4-to-5-half-life clock.
Assuming steady state means a constant level; peaks and troughs continue, they just repeat.
Running the arithmetic with a half-life that has no human source.

Sources

Prescribing informationOZEMPIC (semaglutide) injection, for subcutaneous use: Prescribing Information. Novo Nordisk (FDA-approved labeling), 2026.
Prescribing informationMOUNJARO (tirzepatide) injection, for subcutaneous use: Prescribing Information. Eli Lilly and Company (FDA-approved labeling), 2026.
ReviewPlasma terminal half-life. J Vet Pharmacol Ther, 2004. PMID 15601438.

Page updated October 3, 2026. How we grade evidence: methodology.

Related Terms

Half-Life

Half-life (t½) is the time it takes for the concentration of a drug in your blood plasma to fall by half.

Loading Phase

A loading phase is a period of larger initial dosing meant to reach a target drug level quickly, before switching to a smaller maintenance dose.

Cmin

Cmin is the lowest concentration of a drug in your blood during a dosing interval, usually reached just before the next dose (the trough).

Cmax

Cmax is the highest concentration a drug reaches in your blood after a dose; Tmax is how long after the dose that peak occurs.

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