The science behind the burn-down charts
PeptideBuddy estimates how much of a compound is still active in your body using standard pharmacokinetic (PK) models. Here's exactly what the math does — and its limits.
1. Half-life and first-order elimination
Most drugs are cleared by first-order kinetics: a constant fraction is eliminated per unit time. The amount remaining follows exponential decay:
where t½ is the elimination half-life and k is the elimination rate constant. After each half-life, half of what was present is gone. A practical rule of thumb: a drug is ~97% cleared after 5 half-lives.
| Half-lives | Remaining |
|---|---|
| 1 | 50.0% |
| 2 | 25.0% |
| 3 | 12.5% |
| 4 | 6.3% |
| 5 | 3.1% |
2. Absorption — why levels rise before they fall
Injected and oral compounds aren't instantly in your bloodstream — they're absorbed over time. We model this with the Bateman function (one-compartment, first-order absorption & elimination):
F is bioavailability, D the dose, kₐ the absorption rate constant (derived from the time-to-peak, Tmax), and k the elimination rate. The curve climbs to a peak at Tmax, then decays. When Tmax is unknown we fall back to pure exponential decay.
Both show a single 100-unit dose. Note how the long-half-life compound lingers for weeks while the short one is gone in a couple of days.
3. Multiple doses and accumulation
Real regimens involve repeated doses. Under the linear-PK assumption, the total is simply the superposition (sum) of each dose's individual curve. If you dose again before the previous dose clears, the compound accumulates toward a steady state:
where τ is the dosing interval. For a weekly compound with a ~7-day half-life, R ≈ 1.98× — peak levels at steady state are roughly double a single dose, reached after ~4.9 weeks.
Each week's dose stacks on the residue of previous ones; the troughs and peaks climb until input and elimination balance out.
4. What PeptideBuddy computes for you
- The active amount on board right now, summed across all your logged doses.
- A burn-down curve with a forward projection of when you'll be effectively cleared (<5%).
- How many half-lives have elapsed since your last dose.
- Relative body load as a % of a single dose, so different compounds are comparable.
Important limitations
These are simplified, single-compartment models. They ignore inter-individual variability, renal/hepatic function, non-linear (saturable) kinetics, active metabolites, tissue distribution, and assay differences. Half-lives for research peptides are often poorly characterized estimates. This tool is for education only and is not medical advice. Always consult a qualified clinician about your therapy.