About Estimated Medication Levels
Last updated: 14 April 2026
This page explains how Needled calculates the estimated medication levels you see in the app, where the underlying data comes from, and — just as importantly — what the feature cannot tell you.
What this feature is
The Estimated Medication Levels chart in Needled shows an approximation of how much of your medication is in your system at any given time, based on:
- The doses you've logged in the app
- The medication you've told us you're taking
- Published pharmacokinetic data from clinical studies and official prescribing information
It's a visual tool to help you understand the general rhythm of your medication cycle — when levels are building, when they're peaking, and when they're declining before your next dose.
What this feature is not
Before going any further, a few important things to be clear about:
It is not a blood test. The chart does not measure anything in your body. It does not know your actual medication concentration. It is a mathematical model applied to the doses you've logged.
It is not personalised to you. The chart is built from population averages — the "typical" way these medications behave across thousands of people in clinical trials. Your own biology may produce quite different results.
It is not medical advice. Needled is not a medical device, and this feature is not a diagnostic or treatment tool. It should never be used to make decisions about changing your dose, skipping a dose, timing an injection differently, or adjusting your treatment in any way. Always talk to your doctor or prescriber about your medication.
It is not a substitute for professional guidance. If you have questions about how your medication is working, how you're feeling, or whether your treatment is right for you, please speak to a qualified healthcare professional.
Why individual results vary so much
Peer-reviewed research has consistently shown that there is a 2 to 3 fold variation in blood concentrations between different people taking exactly the same dose of a GLP-1 medication. That's a significant range, and it's influenced by factors including:
- Body weight and composition
- Individual metabolism and genetics
- Injection site and technique
- How much of the medication binds to proteins in your blood (both semaglutide and tirzepatide are over 99% protein-bound, and only the unbound fraction is active)
- Individual sensitivity of the receptors the medication acts on
Clinical pharmacologists who have studied these medications have made this point repeatedly: no clear, predictable relationship between blood concentration and clinical effect has been established for weight loss. Some people lose significant weight at very low doses. Others need maximum doses to see results. This isn't because the medication is different — it's because people are different.
All of this means that any chart showing "your medication levels" is, at best, showing you what levels would look like in an average person taking your dose schedule. It's useful for understanding the general shape of your medication cycle, but it cannot predict your individual experience.
How the calculation works
Needled uses a standard pharmacokinetic model called a one-compartment model with first-order absorption and elimination. This is the same type of model used in published pharmacokinetic research and by pharmaceutical regulators to describe how medications move through the body.
In plain terms, the model describes three phases that happen after every injection:
- Absorption — After you inject, the medication is gradually absorbed from the injection site into your bloodstream, reaching a peak concentration some hours or days later.
- Peak — The medication reaches its highest level in your body.
- Elimination — Your body slowly clears the medication, with the level dropping by half over a period called the "half-life."
For each medication Needled supports, we use three key parameters published in FDA prescribing information and peer-reviewed clinical studies:
- Half-life — How long it takes for your body to clear half of the medication
- Time to peak (Tmax) — How long it takes after injection to reach maximum concentration
- Bioavailability — How much of the injected dose actually reaches your bloodstream
When you log multiple doses, the model adds up the contribution from each one that hasn't yet fully cleared — this is how the chart shows levels accumulating over your first few weeks on a medication as you approach what clinicians call "steady state."
The parameters we use
| Medication | Active ingredient | Half-life | Time to peak | Bioavailability |
|---|---|---|---|---|
| Ozempic | Semaglutide | ~7 days | 30–56 hours | ~89% |
| Wegovy | Semaglutide | ~7 days | 30–56 hours | ~89% |
| Mounjaro | Tirzepatide | ~5 days | 8–72 hours (median 24h) | ~80% |
| Zepbound | Tirzepatide | ~5 days | 8–72 hours (median 24h) | ~80% |
| Saxenda | Liraglutide | ~13 hours | 8–12 hours | ~55% |
| Victoza | Liraglutide | ~13 hours | 8–12 hours | ~55% |
| Trulicity | Dulaglutide | ~5 days | 24–72 hours | 47–65% |
For the time to peak, we use representative median values in the calculation. The wide ranges shown above (particularly the 8 to 72 hour range for tirzepatide) are one of the reasons estimated levels can only ever be an approximation.
Our sources
The following peer-reviewed studies, regulatory documents, and clinical references are the basis for the pharmacokinetic parameters and modelling approach used in Needled.
Semaglutide (Ozempic, Wegovy)
- Clinical Pharmacokinetics of Semaglutide: A Systematic Review. Drug Design, Development and Therapy, 2024. A systematic review of 17 studies covering semaglutide pharmacokinetics in both healthy subjects and patients. PMC link
- Pharmacokinetics and Clinical Implications of Semaglutide. PubMed, 2018. Key reference establishing the approximately 7-day half-life of subcutaneous semaglutide. PubMed link
- FDA Ozempic Prescribing Information (NDA 209637). The official label issued by the US Food and Drug Administration, containing the clinical pharmacology section used in our model. FDA link
- Clinical Pharmacokinetics of Oral Semaglutide: Analyses of Data from Clinical Pharmacology Trials. Clinical Pharmacokinetics (Springer), 2021. Population pharmacokinetic model built from six clinical pharmacology trials. Springer link
Tirzepatide (Mounjaro, Zepbound)
- Population Pharmacokinetics of the GIP/GLP Receptor Agonist Tirzepatide. PMC, 2024. Two-compartment population pharmacokinetic model developed from 19 clinical studies. PMC link
- Tirzepatide. StatPearls, NCBI Bookshelf, 2024. Comprehensive peer-reviewed monograph covering tirzepatide pharmacology, dosing, and clinical use. NCBI link
- FDA Mounjaro Prescribing Information (NDA 215866). The official FDA label for Mounjaro, including clinical pharmacology data. FDA link
- FDA Zepbound Prescribing Information. The official FDA label for Zepbound (tirzepatide for weight management), published by Eli Lilly. Eli Lilly link
- Absorption, Distribution, Metabolism, and Excretion of Tirzepatide in Humans, Rats, and Monkeys. ScienceDirect, 2024. Radiolabelled ADME study of tirzepatide in healthy human participants. ScienceDirect link
Cross-medication references
- A Comprehensive Review on the Pharmacokinetics and Drug-Drug Interactions of Approved GLP-1 Receptor Agonists and a Dual GLP-1/GIP Receptor Agonist. Drug Design, Development and Therapy, 2024. Comparative pharmacokinetic data across the full GLP-1 receptor agonist class. Dovepress link
Liraglutide (Saxenda, Victoza) and Dulaglutide (Trulicity)
Parameters for liraglutide and dulaglutide are drawn from the FDA prescribing information for each medication and the comparative review cited in reference 10 above.
A note from Needled
We built this feature because our users kept asking for it — and because the only other apps that offered it were charging premium subscription fees for something that's based on publicly available pharmacokinetic data. We believe that information should be free.
But we also want to be upfront about what this feature can and can't do. A chart cannot replace a conversation with your doctor. A mathematical model cannot account for your individual biology. And no app can tell you how your medication is really working inside you — only how you feel, the changes you see over time, and guidance from your healthcare team can do that.
Use the chart as a tool for curiosity and understanding. Use it to recognise patterns in how you feel throughout your medication cycle. Use it to have better-informed conversations with your prescriber. But please don't use it to make medical decisions on your own.
If you have any questions about this feature or would like to share feedback, please get in touch at support@needled.app.
Disclaimer
Needled is a tracking and educational tool. It is not a medical device. The Estimated Medication Levels feature provides visualisations based on mathematical models derived from published pharmacokinetic data. These estimates reflect population averages and do not account for individual variations in metabolism, body composition, injection technique, or other factors. This feature is provided for informational and educational purposes only. It does not measure, diagnose, or predict your actual medication blood levels. You should not use this feature to make decisions about your medication dosage, timing, or treatment plan. Always consult a qualified healthcare provider regarding your medication.