It is the single most asked question in this category, and it always arrives in the same shape: I’m at 15mg tirzepatide, where do I start on retatrutide? The expected answer is a number. The honest answer is that no such number exists — not because nobody has bothered to work it out, but because the two compounds are not doing the same thing to the same receptors, and there is nothing to convert.
That answer sounds evasive until you see why it holds. Then it becomes the most useful thing anyone can tell you, because it redirects you from a fake calculation toward the approach the trials and the experienced end of the community actually use.
Four compounds, four different jobs
The word GLP-1 has become a category label, and that label is doing a lot of damage here. It implies these molecules are variations on one theme, differing mainly in strength. They are not.
| Compound | Receptors engaged | What that means |
|---|---|---|
| Semaglutide | GLP-1 only | The single-pathway benchmark |
| Tirzepatide | GIP + GLP-1 | Dual incretin |
| Retatrutide | GIP + GLP-1 + glucagon | Triple; adds an energy-expenditure arm |
| Cagrilintide | Amylin (and calcitonin) receptors | Not an incretin at all |
Cagrilintide is the clearest illustration. It sits in the same conversation, gets stacked with the same compounds, and produces overlapping effects — but it works through the amylin system, a separate hormonal axis. Asking what dose of cagrilintide equals 15mg tirzepatide is like asking how many kilometres are in a kilogram. The units do not map because the mechanisms do not overlap. Our amylin analogs explainer covers why that pathway is a second lever rather than a variation on the first.
The incretins are subtler but the same problem applies. Moving from tirzepatide to retatrutide, you keep GIP and GLP-1 and add glucagon-receptor activity. You are not scaling a dose up. You are switching to a molecule that recruits a pathway the previous one never touched — and the reverse move, retatrutide to tirzepatide, drops that pathway entirely.
Three reasons equivalence can’t transfer
Different receptor sets. Covered above, and it is the biggest one. Even between semaglutide and tirzepatide — the closest pair here, and the one most people assume converts cleanly — the GIP arm is unique to tirzepatide. There is no semaglutide dose that produces GIP agonism.
Different potency per milligram. A milligram is a unit of mass, not of effect. These molecules differ in receptor binding affinity, in how much of each dose reaches circulation, and in what a given plasma concentration does at each target. Semaglutide’s studied maintenance range tops out at 2.4mg weekly; tirzepatide’s at 15mg. That is a roughly sixfold difference in mass at the top of each range, and it tells you almost nothing about relative potency — it tells you these are differently sized molecules with differently scaled dose-response curves.
Different half-lives. Semaglutide and tirzepatide both run around a week. Retatrutide is broadly similar. Cagrilintide sits near 7-8 days, and newer amylin analogs stretch to two weeks. Half-life governs how much compound accumulates before the level plateaus, and therefore what any given weekly dose actually produces at steady state. Two compounds at the same nominal weekly mg with different half-lives are not at the same exposure. The half-life plotter shows this behaviour directly.
What the research programmes actually did
Here is the part that resolves the question. The trials had this problem too — many participants had prior incretin exposure — and they did not solve it with a conversion table. They solved it by starting the new compound at its own starting dose and titrating from there, exactly as they would for someone who had never taken anything.
That is the answer to where do I start on retatrutide. You start at retatrutide’s starting dose. Your 15mg of tirzepatide is not a credit you carry over. It is a different molecule’s number, and it does not denominate anything on the new scale.
- 1
Stop the old compound
Its own half-life governs washout. With a roughly one-week half-life, meaningful levels persist for several weeks after the last dose — the tail overlaps the start of the new compound, which is part of why early tolerance often looks good.
- 2
Start the new compound at ITS starting dose
Not a converted dose. Retatrutide's studied programmes began at 1-2mg weekly regardless of what came before. Tirzepatide begins at 2.5mg. Semaglutide at 0.25mg.
- 3
Hold, then escalate on the new compound's schedule
Four weeks per step is the pattern the trials used. Prior incretin exposure often means you tolerate this better than a true beginner — but the schedule is the schedule.
- 4
Judge the new compound on its own response
Appetite effect at a given step tells you where you are. The old compound's dose number never enters the calculation.
The one thing prior exposure does buy you
It is not a dose credit. It is a tolerance credit, and it is real.
The gastrointestinal side effects that dominate incretin titration — nausea, reflux, altered bowel habit — concentrate during escalation and are largely a function of how fast the system is being pushed into unfamiliar territory. Someone arriving at retatrutide from a year on tirzepatide is not arriving naive. Gastric emptying is already adapted. The satiety signalling is not a novel input. In practice this means escalation frequently feels smoother than the first time round, and people often move through the low steps without the rough patch a beginner gets.
What it does not mean is that you can skip steps and land at some notionally equivalent dose. It means the standard schedule tends to be comfortable rather than punishing. That is a quality-of-life difference, not a licence to start at the top. And the glucagon arm on retatrutide is genuinely new territory even for a tirzepatide veteran — prior incretin exposure buys you nothing there, because nothing you took before engaged it.
For the specific semaglutide-to-tirzepatide move, which has enough shared mechanism and enough real-world history to discuss on its own terms, see switching from semaglutide to tirzepatide. This article is the general problem; that one is the single best-characterised case of it.
The studied ranges, per compound
These are the doses the trials used. They are not converted from each other, and they are not meant to be lined up side by side — read each column on its own.
| Compound | Studied starting dose | Escalation | Studied maintenance range |
|---|---|---|---|
| Semaglutide | 0.25mg weekly | 4-week steps | 1.7-2.4mg weekly (obesity trials) |
| Tirzepatide | 2.5mg weekly | 4-week steps | 5, 10, 15mg weekly |
| Retatrutide | 1-2mg weekly | 2-4 week steps | 4, 8, 12mg weekly (phase 2 arms) |
| Cagrilintide | 0.25mg weekly | 4-week steps | up to 2.4mg weekly |
Notice that cagrilintide’s top studied dose and semaglutide’s top studied dose are the same number — 2.4mg — while the compounds share no mechanism whatsoever. That coincidence is a good vaccine against reading mg columns across rows. The number 2.4 means something different in each row, and nothing at all between them.
Per-compound titration detail lives in the dedicated pieces: retatrutide, tirzepatide, semaglutide and cagrilintide. When you land on a new compound, the reconstitution calculator handles the part that genuinely is arithmetic — turning a target dose into syringe units for your vial size — which is worth doing carefully, because a new compound usually means a new vial strength and a new units-per-dose that looks nothing like the number your hand is used to.
Why the question keeps getting asked anyway
Because the alternative feels like a loss. You spent months titrating to 15mg. Being told that number does not transfer reads as being told to start over — as if the progress is forfeit.
It isn’t, and reframing helps. What you built over those months was not a dose. It was an adapted gut, a calibrated sense of what appetite suppression feels like at a given level, and the practical fluency to run a titration. All three transfer completely. The only thing that does not transfer is the integer, and the integer was never the asset.
Frequently Asked Questions
What dose of retatrutide equals 15mg of tirzepatide?
No dose does, and no published research establishes one. Retatrutide adds glucagon-receptor agonism that tirzepatide does not have, so the compounds are not on a shared scale. The approach the trials used is to start retatrutide at its own studied starting dose — 1-2mg weekly — and titrate on its own schedule, regardless of prior tirzepatide exposure.
Can I skip the low doses if I have been on a GLP-1 for a year?
Prior incretin exposure usually means better gastrointestinal tolerance, so escalation tends to feel smoother than it did the first time and there is less reason to be ultra-cautious at each step. That is different from skipping steps. The studied schedules escalate in 2-4 week increments because that is how long a compound with a roughly one-week half-life takes to express a dose change at steady state — which is a pharmacokinetic fact, not a tolerance concession.
Why can’t a conversion table just be built from the trial data?
Because the trials measured different compounds in different populations over different durations with different titration schedules. Dividing one trial’s weight-loss percentage by its dose and comparing to another’s produces a number, but the number describes two unrelated experiments, not a relationship between the molecules. Cross-trial arithmetic is not equivalence, and only a head-to-head study designed to establish it could be — none has been published.
Does cagrilintide convert to a GLP-1 dose at all?
No, and this is the cleanest case. Cagrilintide is an amylin receptor agonist. It does not engage GLP-1 or GIP receptors. There is no exchange rate between an amylin dose and an incretin dose because they are not the same currency — they are separate hormonal axes that happen to produce overlapping effects on appetite.
How long should I wait between stopping one and starting another?
The old compound’s half-life governs this. With roughly one-week half-lives, meaningful levels persist for several weeks after a final dose, so there is no sharp boundary — the tail of the old compound overlaps the start of the new one either way. In practice most switches simply begin the new compound at the next scheduled injection day, which is also part of why the first few weeks on the new compound often feel deceptively easy.
Related reading
- Switching from Semaglutide to Tirzepatide: A Research Guide — the one pair with enough history to discuss specifically
- Retatrutide vs. Tirzepatide: What the 2026 Research Shows — the receptor and trial-data comparison behind this article
- Peptide Half-Life Explained: The Number Behind Dosing — why weekly mg alone does not describe exposure
- Amylin Analogs Explained — the axis that does not convert to anything incretin