Free Tool · One-Compartment PK Model · 30+ Compounds
Select a compound, enter your dose and dosing interval, and instantly see the full plasma accumulation curve — from first injection to steady state. Covers semaglutide, tirzepatide, retatrutide, BPC-157, and 30+ injectable compounds with citation-backed half-life data.
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The following table summarizes elimination half-life data for major injectable compounds. All GLP-1 values are sourced from FDA prescribing information or Phase 3 clinical pharmacology data. Research peptide values are sourced from peer-reviewed preclinical pharmacokinetic studies.
| Compound | Category | Half-Life (t½) | Dosing Interval | Doses to SS | Source |
|---|---|---|---|---|---|
| Semaglutide | GLP-1 agonist | 7 days (168 h) | Weekly | ~5 doses | FDA NDA 209637 |
| Tirzepatide | GIP/GLP-1 dual agonist | 5 days (120 h) | Weekly | ~4–5 doses | FDA NDA 215866 |
| Retatrutide | GIP/GLP-1/Glucagon triagonist | 6 days (144 h) | Weekly | ~4–5 doses | Phase 3 (NCT05929261) |
| Liraglutide | GLP-1 agonist | 13 hours | Daily | ~3 days | FDA NDA 022341 |
| CJC-1295 DAC | GHRH analogue | 8 days (192 h) | 2× monthly | ~4–5 doses | Teichman et al. 2006 |
| Testosterone Enanthate | Androgen | 4.5 days (108 h) | 1–2× weekly | ~4 weeks | FDA label |
| MK-677 (Ibutamoren) | GH secretagogue | 24 hours | Daily | ~5 days | Nass et al. 2008 |
| TB-500 (Thymosin β4) | Repair peptide | 12 hours | 2× weekly | ~2–3 days | Preclinical PK |
| BPC-157 | Repair peptide | 4 hours | 2× daily | ~1 day | Preclinical PK |
| Ipamorelin | GHRP (GH secretagogue) | 2 hours | 2–3× daily | ~12 hours | Raun et al. 1998 |
| CJC-1295 (no DAC) | GHRH analogue | 30 minutes | 2–3× daily | ~2.5 hours | Ibid. |
| Sermorelin | GHRH(1-29) | 10 minutes | Daily (bedtime) | <1 hour | FDA label (Geref) |
| PT-141 (Bremelanotide) | Melanocortin agonist | 2 hours | As needed | ~10 hours | FDA NDA 022358 |
| MOTS-C | Mitochondrial-derived peptide | 4 hours | Weekly | ~1 day | Lee et al. 2015 |
| Epitalon | Telomerase activator | 3 hours | Daily | ~15 hours | Khavinson 2012 |
| GHK-Cu | Copper peptide | 1 hour | Daily | ~5 hours | Pickart et al. 2018 |
| IGF-1 LR3 | Growth factor analogue | 24 hours | Daily | ~5 days | Francis et al. 1992 |
The elimination half-life (t½) of a drug is the time required for its plasma concentration to decrease by 50% after a single dose. For compounds that follow first-order elimination kinetics — the standard model for GLP-1 agonists and most injectable peptides — this decay is exponential:
This has a critical implication: a compound is never truly "zero" — it decays asymptotically. The conventional threshold for "fully cleared" is 5 half-lives, at which point ~97% of the drug has been eliminated. For semaglutide (t½ = 7 days), that means 35 days after the last injection before plasma levels fall to near-zero.
When a drug is dosed repeatedly before it fully clears, each injection adds to the residual concentration from the previous dose — a process called drug accumulation. The plasma concentration climbs dose-over-dose until the rate of elimination exactly matches the rate of input. This equilibrium point is called steady state (SS).
The mathematical rule: steady state is reached after approximately 5 elimination half-lives of dosing, at which point Cmax and Cmin are ~97% of their final values and remain constant from dose to dose.
| Compound | t½ | Time to Steady State | SS Peak (Accumulation) |
|---|---|---|---|
| Semaglutide | 7 days | ~35 days (5 wk) | ~2× single-dose peak |
| Tirzepatide | 5 days | ~25 days (4 wk) | ~1.8× single-dose peak |
| Retatrutide | 6 days | ~30 days (4–5 wk) | ~1.9× single-dose peak |
| MK-677 | 24 hours | ~5 days | ~1.4× single-dose peak |
| BPC-157 | 4 hours | ~20 hours | ~1.01× (near zero accumulation) |
This calculator uses a one-compartment pharmacokinetic model, which assumes the body behaves as a single well-mixed compartment and that absorption is instantaneous (IV-equivalent). It computes concentration at time t as the sum of contributions from all prior doses:
Real-world pharmacokinetics are more complex: subcutaneous absorption is not instantaneous (there is a Tmax delay), distribution occurs across multiple compartments, protein binding affects free drug fraction, and non-linear kinetics can emerge at high concentrations. These factors mean the tool is a directionally accurate approximation, not a clinical-grade simulator — but for the purposes of understanding accumulation timing and trough/peak ratio, the one-compartment model is the established standard reference.
For a compound with a half-life close to or longer than its dosing interval, a meaningful fraction of each prior dose is still circulating when the next dose is injected. Consider semaglutide dosed weekly with a 7-day half-life: by the time dose 2 is administered, exactly 50% of dose 1 remains. Dose 3 is administered on top of 25% of dose 1 and 50% of dose 2. This compound residual accumulates geometrically until equilibrium is reached.
The accumulation factor at steady state is given by:
This means semaglutide's steady-state peak is exactly 2× the single-dose peak. Tirzepatide (t½ = 5 days, τ = 7 days) has R ≈ 1.81 — about 81% above the first-dose level. This is why the 4th and 5th weeks of a new GLP-1 dose level feel meaningfully stronger than the first week.
The accumulation effect is the pharmacokinetic foundation of dose escalation schedules. Starting tirzepatide directly at 5 mg (the second escalation step) would expose the patient to steady-state concentrations equivalent to ~9 mg by week 4 — approximately the fourth dose step — before their GI system has adapted. The FDA-mandated escalation (2.5 mg × 4 weeks → 5 mg × 4 weeks → 7.5 mg…) is designed so that each 4-week interval allows the patient to experience full steady-state at that dose before escalating. The calculator on this page models exactly this phenomenon.
The trough-to-peak ratio (T/P ratio, expressed as %) is the plasma concentration just before the next dose divided by the peak just after dosing. It is a direct measure of plasma concentration fluctuation within each dosing cycle.
All three are once-weekly subcutaneous injections, but their pharmacokinetic profiles differ in ways that affect clinical experience — particularly the timing of maximum effect and the duration of detectable drug levels after discontinuation.
| Property | Semaglutide | Tirzepatide | Retatrutide |
|---|---|---|---|
| Mechanism | GLP-1 mono-agonist | GIP + GLP-1 dual | GIP + GLP-1 + Glucagon triple |
| Half-life (t½) | 7 days (168 h) | 5 days (120 h) | 6 days (144 h) |
| Time to steady state | ~5 weeks | ~4 weeks | ~4–5 weeks |
| SS accumulation factor | ~2.0× | ~1.81× | ~1.90× |
| Trough/peak ratio | ~50% | ~45% | ~47% |
| Full clearance after last dose | ~35 days | ~25 days | ~30 days |
| Maximum approved dose | 2.4 mg/wk (Wegovy) | 15 mg/wk (Zepbound) | 12 mg/wk (Phase 3) |
| Starting dose | 0.25 mg × 4 wk | 2.5 mg × 4 wk | 2 mg × 4 wk |
| FDA approval status | Approved (NDA 209637) | Approved (NDA 215866) | Phase 3 (investigational) |
Semaglutide has a 7-day (168 h) elimination half-life. Following first-order kinetics, plasma levels halve every 7 days after the last dose: 50% remaining at day 7, 25% at day 14, 12.5% at day 21, and less than 3% by day 35. In practice, measurable appetite suppression typically diminishes within 2–4 weeks of the last injection, and most clinical trial protocols consider semaglutide pharmacologically inactive ~5 weeks post-dose. Weight regain data from STEP 4 showed an average of 0.5–1.0 kg per week during the first 8 weeks after discontinuation.
Tirzepatide has a 5-day (120 h) elimination half-life per FDA NDA 215866. Using the 5 half-life clearance rule: 50% at day 5, 25% at day 10, 12.5% at day 15, ~3% at day 25. Tirzepatide is substantially cleared within 25 days of the last dose. Because its half-life is shorter than semaglutide's, tirzepatide also clears faster between doses — its trough-to-peak ratio (~45%) is slightly lower, meaning slightly more fluctuation within each week compared to semaglutide (~50%).
Tirzepatide (Zepbound / Mounjaro): Per the FDA prescribing information, if the missed dose is within 4 days of the scheduled day, administer it as soon as possible and resume the normal weekly schedule. If more than 4 days have passed, skip the missed dose entirely and inject on the next regularly scheduled day.
Semaglutide (Wegovy / Ozempic): If the missed dose is within 5 days of the scheduled day, administer it immediately. If more than 5 days have passed, skip and resume the regular schedule. Never take two doses in the same week to compensate. Due to the long half-life, a single missed dose reduces plasma concentration by less than 10% — clinically, the impact of one missed weekly injection is minor.
Tirzepatide reaches steady state after approximately 4–5 weeks of consistent weekly dosing (4–5 × t½ = 4–5 × 5 days = 20–25 days). At steady state, the Cmax is approximately 1.81× the first-dose Cmax. This accumulation is why side effects — nausea, appetite suppression, fatigue — typically reach maximum intensity during weeks 4–6 of a new dose level. After week 5, concentrations plateau and side effects often stabilize or begin to attenuate as tolerance develops.
Tirzepatide: t½ ≈ 5 days (120 h). Semaglutide: t½ ≈ 7 days (168 h). The 2-day difference creates measurable pharmacokinetic distinctions: semaglutide accumulates to a higher steady-state multiple (~2.0× vs ~1.81×), takes ~1 additional week to reach steady state, maintains a slightly higher trough-to-peak ratio (meaning more stable week-to-week plasma levels), and takes about 10 additional days to clear completely after discontinuation. In clinical practice, these differences are modest — both exhibit the long-acting, high-trough profile that distinguishes weekly GLP-1s from daily formulations like liraglutide.
Because that is when plasma concentrations first reach steady state — the maximum accumulation level. At the first injection, you are experiencing a single-dose Cmax. By week 4–5, the steady-state Cmax is approximately 1.8–2.0× higher than that first dose. Every GLP-1 receptor exposed to the drug (GI tract motility receptors, central appetite circuits, gastric emptying regulation) is now experiencing roughly double the drug concentration that it encountered in week 1.
This is the pharmacokinetic rationale for the standard 4-week minimum between dose escalation steps: each new dose needs a full month to reach its new steady state before the protocol moves to the next level. Escalating faster stacks accumulation cycles and dramatically increases side-effect burden.
At steady state, the amount of tirzepatide or semaglutide you eliminate in the 7 days between injections exactly equals the dose you inject. Plasma concentration reaches the same peak and trough every single week — the curve has "flattened." Before steady state is reached, your plasma concentration is still rising each week. Think of it like filling a bathtub where the drain is partially open: the water level rises week over week until drain rate = fill rate. Your 5th or 6th injection is the point where the water level stops rising.
BPC-157 (Body Protection Compound 157) has a reported elimination half-life of approximately 4 hours in preclinical rodent pharmacokinetic studies. Using the 5 half-life rule, it is ~97% cleared within 20 hours of the last injection. This extremely short half-life explains the standard twice-daily dosing protocol (every 12 hours) — at 12h intervals with a 4h half-life, trough levels are only ~7% of peak, meaning nearly complete elimination between doses. BPC-157 does not accumulate meaningfully, even with twice-daily dosing.
Ipamorelin has a half-life of approximately 2 hours. CJC-1295 without DAC has a half-life of approximately 30 minutes. CJC-1295 with DAC (Drug Affinity Complex) has a dramatically extended half-life of approximately 8 days (192 hours) due to albumin binding. The ipamorelin + CJC-1295 no-DAC combination is specifically designed to leverage their complementary mechanisms (GHRP + GHRH) with matched short half-lives that produce a single, clean GH pulse — mimicking endogenous pulsatile GH secretion and avoiding the receptor desensitization associated with continuous GH axis stimulation.
From a purely pharmacokinetic standpoint, missing a single weekly dose causes your plasma concentration to fall by approximately one half-life's worth of decay below the normal trough level. For semaglutide (t½ = 7 days), plasma levels at the end of a 14-day gap (double the normal interval) are approximately 50% of the usual pre-dose trough — a meaningful but not dramatic reduction. Clinically, many patients notice increased appetite within 1–2 weeks of a missed dose, particularly if they were at steady state. Resume the normal schedule on the next scheduled day; do not double-dose.
Related Compound Pages
Simplified one-compartment model assumes instantaneous subcutaneous absorption and linear first-order elimination. Does not model volume of distribution, protein binding, enterohepatic recirculation, or non-linear (Michaelis-Menten) kinetics. For educational and informational reference only — not a substitute for clinical pharmacokinetic analysis or medical advice. Half-life values sourced from FDA prescribing information and peer-reviewed literature; research peptide values from preclinical studies and may not reflect human pharmacokinetics. Methodology · Terms