Animal-to-Human Dose Conversion for Peptides Explained
Why a 10 mcg/kg rat study does not mean you take 10 mcg/kg — and how the conversion actually works.
Abstract: Almost every peptide efficacy study you will read was run in rats or mice, yet the mg/kg number on the page cannot be used directly in a human. This guide walks through animal-to-human dose conversion for peptides — the FDA body-surface-area method, the route adjustment, and the sanity checks — with worked examples for six common peptides.
If you have ever read a peptide paper, seen “10 mcg/kg” in the methods, and quietly multiplied it by your bodyweight, this guide is for you. Animal-to-human dose conversion is the single most misunderstood step in reading peptide research, and getting it wrong is how people end up either wildly overdosing or dismissing a compound that was actually tested at a sensible exposure. The good news: there is a standard, published method, it is not complicated, and once you know it you can read almost any animal study critically. The important caveat up front — this is a tool for understanding the literature, not a dosing prescription. Peptides discussed here are sold and studied for research and educational use only.
Why animal-to-human dose conversion isn’t just multiplying mg/kg
The intuitive move — take the rat’s mg/kg and apply it to yourself — is wrong, and it is wrong in a specific, predictable direction. Drug exposure does not scale with bodyweight; it scales with body surface area. Metabolic rate, cardiac output, and renal clearance all track surface area far more closely than mass across species (Nair & Jacob, 2016).
A rat has far more surface area per kilogram than a human does, so it burns through a compound faster. To reach the same internal exposure, a small animal needs a higher mg/kg dose than a human. If you copy the rat’s mg/kg straight across, you overshoot — often by six-fold or more. That is the entire reason a formal conversion exists.
Step 1 — Allometric scaling (the FDA Kₘ method)
The U.S. FDA recommends a body-surface-area normalization for estimating a human starting dose in first-in-human trials, and the same math lets you read animal studies in reverse (FDA, 2005; Reagan-Shaw et al., 2008). Each species gets a Kₘ factor — its average bodyweight divided by its surface area:
| Species | Kₘ | Shortcut ×factor → human |
|---|---|---|
| Mouse | 3 | ×0.081 |
| Rat | 6 | ×0.162 |
| Rabbit | 12 | ×0.324 |
| Dog | 20 | ×0.541 |
| Human (~60 kg) | 37 | — |
The formula is:
HED (mg/kg) = Animal dose (mg/kg) × (Kₘ animal ÷ Kₘ human)
For a rat that is 6 ÷ 37 ≈ 0.162 — the multiplier you will see again and again in serious peptide write-ups. So a 10 mcg/kg rat dose becomes ~1.62 mcg/kg human-equivalent before any other adjustment. The same logic confirms the widely cited rat-to-human ratio of roughly 1:6.2.
Step 2 — Adjust for route and bioavailability
Allometric scaling gets you to a human-equivalent exposure, but exposure depends on how the drug was delivered. Most animal peptide studies inject intraperitoneally (IP) — into the abdominal cavity — which is absorbed roughly 40% more completely than a subcutaneous (SubQ) injection, the route most human research and self-experimentation actually uses.
So to match the IP exposure with a SubQ shot, you scale up: SubQ dose ≈ HED × 1.4. Other rules of thumb worth carrying: IV = 100% bioavailability, SubQ ≈ 70% of IV, and oral varies enormously — acid-resistant oral BPC-157 lands around half of its SubQ dose, while most other peptides survive the gut far worse and need much more (or a different route entirely). Half-life matters here too: short-acting peptides need their dose split across the day rather than given all at once, a point that drives most real-world peptide dosing and timing decisions.
Step 3 — Convert per-kg to per-person
Now multiply the route-adjusted mg/kg by a realistic bodyweight — typically 70–80 kg for an adult. This is the step where an abstract mg/kg figure finally becomes a number you can compare against what the human community actually uses.
Step 4 — Sanity-check and apply a safety factor
The estimate is a ceiling for interpretation, not a target. Two checks keep you honest:
First, drug development routinely divides the modelled human dose by ~10 as a safety margin before the first human is ever dosed. Second, compare your result to anecdotal human ranges — which, for many peptides, run higher than the strict allometric estimate because tolerance, receptor dynamics, and half-life differ between species. When the two disagree, that disagreement is information, not an error to paper over. The universal rule that falls out of all of this is start low, titrate slow — see our peptide side effects and safety guide for why conservative titration matters more than hitting a “correct” number.
Worked examples: six peptides, start to finish
These conversions are derived from primary animal studies and physician deep-dives in our research base. They show how the same four steps produce very different — and sometimes surprising — answers.
| Peptide | Animal dose | Conversion | Human-equivalent (modelled) | Anecdotal human range |
|---|---|---|---|---|
| BPC-157 | 10 mcg/kg rat IP | ×0.162 → 1.62, ×1.4 (IP→SubQ) → 2.27 mcg/kg | ~181 mcg/day (80 kg) | 250–500 mcg/day SubQ |
| MOTS-C | 0.5 mg/kg/day IP | scaled + route → ~0.048 mg/kg SubQ | ~3.3 mg/day (~23 mg/week) | ~5 mg, 3×/week |
| TB-500 / TB4 | 6 mg/kg rat IP | ×0.162 → 0.49, ×1.4 → ~0.69 mg/kg | ~48 mg/day (impractically high) → re-anchored to human IV-safety data → ~500 mcg/day | 250–500 mcg/day |
| SS-31 | 3 mg/kg mouse | allometric HED | ~17 mg/day; a 1 mg/kg study → ~6 mg/day | start 5–10 mg/day |
| Selank | 100 mcg/kg/day | ×0.162 → 16.2 mcg/kg → ~1 mg/day, then ÷10 safety | ~100 mcg/day start | 250–900 mcg intranasal |
| GHK-Cu | 0.5 mg/kg EOD IP | allometric + IP→SubQ | ~0.6–1 mg/day SubQ | 0.5–2 mg/day |
Two of these are worth dwelling on, because they teach the method better than the clean cases:
BPC-157 is the textbook example. The rat study used 10 mcg/kg IP; the full conversion lands near 181 mcg/day, and the anecdotal human range (250–500 mcg/day) sits modestly above it — exactly the “humans tend to run a little higher” pattern you should expect. The number is reassuringly sane. For the underlying evidence, see our BPC-157 complete guide and the product page for BPC-157.
TB4 is the cautionary one. Run the raw 6 mg/kg rat dose through the same machinery and you get ~48 mg/day — an absurd figure no one injects. That is not a license to dose to 48 mg; it is a flashing red light that the animal study used an exposure that does not translate, and the only defensible human anchor is separate IV-safety data (~5 mcg/kg), which points to ~500 mcg/day. When allometric scaling produces a number that looks insane, the scaling is telling you the truth: that protocol was never meant to map cleanly onto a person. The same skepticism applies to SS-31 at the high end of its mouse studies, to MOTS-C in metabolic research, and to GHK-Cu and Selank, where the human-validated routes (topical and intranasal, respectively) differ from the injected animal protocols entirely.
A note on provenance: animal ≠ human evidence
The honest framing of every conversion above is that the efficacy data is animal (preclinical) and the human numbers are anecdotal or extrapolated, not validated in controlled trials. Body-surface-area scaling models equivalent exposure; it does not promise equivalent safety, equivalent half-life, or equivalent receptor behavior. A “no observed toxicity” result in a rat is not a human safety clearance. Treat every modelled HED as a way to read the literature critically — never as a recommendation.
FAQ
Why multiply a rat dose by 0.162 specifically? Because the rat’s Kₘ factor is 6 and the human’s is 37, and 6 ÷ 37 ≈ 0.162. That ratio converts the rat’s mg/kg into a human-equivalent mg/kg on a body-surface-area basis.
Is the human-equivalent dose a recommended dose? No. It is a modelling estimate used to interpret a study and to set a conservative starting point in formal drug development (often divided by ~10 first). It is not medical advice or a dosing prescription.
Why do anecdotal human peptide doses often exceed the calculated HED? Differences in half-life, receptor desensitization, and bioavailability between species, plus the fact that community doses are not rigorously validated. When the two diverge, the gap is a prompt to be more cautious, not less.
Do I need to adjust for the injection route? Yes. Most animal studies inject intraperitoneally, which is ~40% more bioavailable than subcutaneous. To match that exposure SubQ, scale the human-equivalent dose up by roughly 1.4×. Oral and intranasal routes change the math far more.
Does this method work for every peptide? The scaling math is general, but the result is only useful when the animal exposure is realistic. When conversion yields an impractical number (as with TB-500), it is a signal to anchor to human safety data instead of the animal dose.
References
- Nair AB, Jacob S (2016). A simple practice guide for dose conversion between animals and human. Journal of Basic and Clinical Pharmacy 7(2):27-31. PMC4804402
- Reagan-Shaw S, Nihal M, Ahmad N (2008). Dose translation from animal to human studies revisited. FASEB Journal 22(3):659-661. PMID 17942826
- U.S. Food and Drug Administration (2005). Guidance for Industry: Estimating the Maximum Safe Starting Dose in Initial Clinical Trials for Therapeutics in Adult Healthy Volunteers.
Research and educational use only. This article describes how doses were used or modelled in published research and community reports; it is not medical advice and is not a recommendation to use any compound. Conversions are interpretive estimates, not validated human doses. Consult a qualified physician before making any health decision.