Methodology
How PepTrak estimates peptide levels.
Published 2026-06-10. This page describes software behavior, cites public drug labeling, and has not been medically reviewed.
The feature
A curve next to the dose card.
What are Estimated Peptide Levels in PepTrak?
For supported GLP-1 protocols, the Today screen shows a curve estimating the amount of compound still active from the doses you logged. The curve gives the record context, like why week one of a protocol can feel different from week five. It is labeled in your logged dose units (for example, milligrams injected), never as a blood concentration.
The model
What the app computes.
How are the estimates calculated?
Each logged dose is assumed to decline by first-order elimination, the standard model behind the published half-life figures: after one half-life, half of a dose's contribution remains; after two, a quarter; and so on. The estimate at any moment is the sum of what remains from every logged dose, so regular weekly dosing shows the familiar accumulation toward a plateau. Prescribing information for once-weekly semaglutide, for instance, describes steady-state exposure after four to five weeks of dosing, which matches the shape the curve takes when doses are logged on schedule.
The model stops there. It does not estimate absorption rate, distribution, injection-site differences, kidney or liver function, body composition, or anything else individual. The whole calculation is one published average half-life per supported compound, applied as exponential decay and summed across logged doses.
The sources
Numbers come from the label.
Where do the half-life values come from?
Half-life values come from FDA prescribing information for the relevant compounds. Two examples that anchor the supported GLP-1 protocols: semaglutide's elimination half-life is approximately one week, and it remains in circulation for about five weeks after the last dose; tirzepatide's elimination half-life is approximately five days. The full source list is below, and the in-app feature carries the same citations and caveats.
- OZEMPIC (semaglutide) Prescribing Information — Clinical Pharmacology (elimination half-life ~1 week; ~5 weeks in circulation after the last dose)
- WEGOVY (semaglutide) Prescribing Information — Clinical Pharmacology
- MOUNJARO (tirzepatide) Prescribing Information — Clinical Pharmacology (elimination half-life ~5 days)
- ZEPBOUND (tirzepatide) Prescribing Information — Eli Lilly
- StatPearls: Semaglutide — pharmacokinetics overview
- StatPearls: Tirzepatide — pharmacokinetics overview
The limits
Read the curve carefully.
What should you never read into the curve?
The curve is not a blood test, and it is not a signal to take, skip, or change a dose. Label averages describe populations; your clearance can differ for reasons the model does not see. If the curve raises a question about timing, missed doses, or side effects, bring it to your clinician with the record in hand.
Methodology FAQ
Common questions.
What do people ask about estimated levels?
Are Estimated Peptide Levels my blood concentration?
No. They are educational estimates in the dose units you logged, computed from published average half-lives. Actual blood concentration depends on absorption, body composition, metabolism, and other individual factors PepTrak does not model.
How long does semaglutide stay in your system?
FDA prescribing information lists semaglutide's elimination half-life at approximately one week, with the drug present in circulation for about five weeks after the last dose. Individual clearance varies; ask your clinician what it means for you.
How long does tirzepatide stay in your system?
FDA prescribing information lists tirzepatide's elimination half-life at approximately five days. As with any average, individual values vary.
Can I use the levels curve to decide my next dose?
No. The curve is context for the record. Dosing, titration, and missed-dose decisions belong with your licensed clinician.
See the feature in context on the GLP-1 tracker page, or start with the glossary if terms like half-life and steady state are new.