How to read an estimated level curve

STACK draws a curve under your logged doses. It is an arithmetic estimate built from one published number, the compound’s elimination half-life. This page explains what the shape means and what it cannot tell you. Nothing here is advice about what or when to take anything.

Half-life

A half-life is the time it takes for the amount of drug in the body to fall by half [1]. Elimination follows a first-order process: after one half-life 50% is gone, after two 75%, after three 87.5%, and “after 4 half-lives elimination and accumulation can be considered essentially complete” [1]. The number comes from clinical pharmacology studies and is printed on the label. Semaglutide’s label states an elimination half-life of approximately 1 week and adds that the drug “will be present in the circulation for about 5 weeks after the last dose” [2]. Tirzepatide’s label gives approximately 5 days [3].

STACK uses the labeled or best-published human value as its reference half-life for each compound, and the compound page tells you which value and which source.

Accumulation and steady state

If you take repeated doses before the previous dose has cleared, the doses stack. The same half-life that governs elimination governs how fast the level builds: 50% of the way to steady state after one half-life, 75% after two, and close to steady state after four [1]. Steady state is the point where each new dose only replaces what was eliminated since the last one, so the curve stops climbing and settles into a repeating pattern [1]. Five half-lives gets you to about 97% of that plateau (that is arithmetic: 1 − 0.5⁵). For semaglutide the label reports steady-state exposure after 4 to 5 weeks of once-weekly use, which lines up with a one-week half-life [2].

How high the plateau sits depends on two things: the half-life and how often doses are given [1]. Dosing every half-life gives an accumulation factor of exactly 2, meaning the steady-state level is twice the level after a single dose [1]. Dose more often than the half-life and it builds higher; less often and it builds less.

Weekly versus daily compounds

Long half-life compounds given once weekly, such as semaglutide or tirzepatide, produce a curve that ramps up over the first month or so and then shows a gentle sawtooth: a rise after each injection and a slow fall over the next seven days [1][2][3]. Short half-life compounds given daily show a much sharper sawtooth and reach their plateau within days. The average level at steady state depends only on the total amount over time; the size of the swings between peak and trough depends on how the interval compares with the half-life [1]. That is why a weekly curve looks like a rolling hill and a daily curve looks like a comb.

Why it is an estimate, not a blood level

STACK’s curve is an estimate from a published half-life, not a blood level. Four things it leaves out:

1. No absorption model. A subcutaneous dose is not in the blood instantly. Semaglutide reaches maximum concentration 1 to 3 days after a dose [2]; tirzepatide’s time to peak ranges from 8 to 72 hours [3]. STACK adds each dose at the moment you log it and lets it decay from there.
2. One half-life for everyone. Labels report population averages. The Humulin R label says plainly that “the time course of action of any insulin may vary considerably in different individuals or at different times in the same individual” and that it depends on dose, injection site, blood supply, temperature, and physical activity [5].
3. One half-life per compound. Most drugs show more than one half-life as they move between blood and tissue, and using the longest reported value can overstate accumulation [4]. STACK uses a single number.
4. Not a measurement. The curve has no idea what is actually in your blood. A lab test does. The curve only shows what the arithmetic implies from the doses you logged.

Treat it as a picture of consistency, not a lab result.

What a missed dose looks like on the curve

If a scheduled dose is not logged, the curve simply keeps falling. For a compound with a one-week half-life, a missed weekly dose means the level at the next scheduled day is about half of where it would have been [1]. When the next dose is logged, the curve steps up again but from a lower point, and it takes a few half-lives to return to the previous plateau [1]. For a daily compound the dip is shallower and recovers faster because the doses are closer together relative to the half-life.

That is all the curve can tell you. The Ozempic and Mounjaro labels each contain a specific missed-dose section, and what to do about a missed dose is a decision for you and your prescriber, not the app [2][3]. If you missed a dose, call your prescriber.

Reading your own curve in STACK

The curve is drawn from the doses you log for each compound, using that compound’s reference half-life. The compound page states the reference half-life and its source, and you can enter your own estimate if a compound has no published human value. Log doses exactly as your prescriber wrote them, with the real date and time. The curve is only as good as the log underneath it.

Try it: Level simulator

See how an estimated level builds and decays over weeks from a published half-life and a schedule. An illustration of first-order decay — not a blood level.

More free tools

Sources

Every URL below was fetched and checked on 2026-09-24. Bracketed numbers in the text link here.

1. ReviewHolford N. Absorption and Half-Life. Transl Clin Pharmacol. 2016;24(4):157-160
2. LabelOzempic (semaglutide) injection — prescribing information, FDA (2023)
3. LabelMounjaro (tirzepatide) injection — prescribing information, FDA (2024)
4. ReviewSahin S, Benet LZ. The Operational Multiple Dosing Half-life. Pharm Res. 2008;25(12):2869-2877 (PMC3581066)
5. LabelHumulin R U-100 (insulin human) — prescribing information, FDA (2015)