Why Retatrutide Needs a Calculator (Not Just a Chart)
Most drugs are straightforward: take a dose, wait for it to clear, take another. Once-weekly injectable drugs with multi-day half-lives work differently.
With a ~6-day half-life and 7-day injection intervals, approximately 53–60% of each dose is still present when the next injection is given. Each injection adds to residual levels from all prior weeks. By weeks 4–5, the total drug load stabilizes at roughly 3–4× the per-injection amount — the steady-state plateau.
This means the same 4 mg injection means something very different in week 1 vs week 5. A calculator that integrates your full injection history is the only way to know where you actually are.
The Pharmacokinetic Model: One-Compartment, First-Order Kinetics
The Retadose calculator uses a one-compartment pharmacokinetic model with first-order absorption and first-order elimination. This is the standard model class used for weekly subcutaneous injectable drugs.
The three key parameters come from Coskun et al. (2022), the published Phase 1 PK study for retatrutide:
- Elimination half-life (t½): ~144–165 hours (~6 days), dose-independent across therapeutic doses — which makes the model reliable across the dose range
- Absorption rate constant (ka): estimated from time-to-peak (Tmax) following subcutaneous injection
- Volume of distribution (Vd): defines the dose-to-concentration relationship
For multiple injections, the total concentration is the sum of all individual injection curves evaluated at their respective elapsed times. This is what the Retadose calculator computes across your entire log.
What "Your Level" Actually Means
The Retadose output is an estimated relative concentration — not a blood-draw result. It represents drug exposure proportional to real plasma concentration. The shape and direction of the curve are pharmacokinetically grounded; the absolute units require a blood draw to calibrate.
What the curve tells you:
- Whether you are still rising toward steady state or have arrived
- How much your level fluctuates between injections (peak-to-trough ratio)
- How quickly your level will fall if you pause dosing or miss an injection
- Where you are relative to a personal therapeutic window you have defined
Therapeutic Window: The Primary Use Case
The PK curve becomes most valuable when combined with self-reported observations: at what level does appetite suppression kick in? Where do side effects emerge? These are your personal therapeutic window boundaries.
Retadose lets you mark these observations and save them as window boundaries. Future dose changes — escalations, holds, reductions — are then evaluated against your window: will the new dose take you through it, below it, or hold you inside it?
What the Retatrutide Calculator Does Not Do
- Provide individualized dosing recommendations
- Adjust for body weight, renal function, or drug interactions
- Predict clinical outcomes (weight loss, HbA1c, etc.)
- Replace the guidance of a prescribing clinician
- Report actual measured plasma concentrations (requires a blood draw)
The model is calibrated to population-average PK parameters from Coskun et al. (2022). Individual clearance rates vary. Tracking your own history and correlating it with your personal responses is what closes that gap.