Retatrutide vs Tirzepatide Dosing Compared

Last updated: June 2026

Tirzepatide is an FDA-approved dual GIP/GLP-1 agonist titrated 2.5 → 5 → 7.5 → 10 → 12.5 → 15 mg once weekly, while retatrutide is an investigational triple GIP/GLP-1/glucagon agonist that is not approved by the FDA or any regulator — its only dose numbers come from phase-2 trials, where it was stepped up to 1, 4, 8, or 12 mg weekly. They are different molecules on different number lines, and the syringe units you draw for either depend on the vial concentration, not on which drug it is.

Have a vial strength and a target dose? Convert any weekly dose to exact mL and U-100 syringe units in seconds.

Tirzepatide calculator → Retatrutide calculator →

TL;DR — key takeaways

Retatrutide is investigational. It has no approved label, no maximum approved dose, and no official titration schedule. Every retatrutide number on this page is a research-trial figure, not a prescription standard.
Tirzepatide is approved. Its weight-management label (Zepbound) defines a fixed staircase from 2.5 mg up to 15 mg weekly, stepping no sooner than every 4 weeks.
Different receptor counts. Tirzepatide hits two receptors (GIP + GLP-1); retatrutide adds a third (glucagon). More targets does not mean the milligram scales line up.
Units follow concentration. Whether you draw 10, 25, or 60 units depends on the mg/mL of your vial — the same arithmetic for both drugs.

Why these two dose scales are not comparable

Tirzepatide is a dual agonist of the glucose-dependent insulinotropic polypeptide (GIP) receptor and the glucagon-like peptide-1 (GLP-1) receptor. Retatrutide adds a third arm: it also activates the glucagon receptor, making it a triple agonist. Because the molecules bind different receptor sets with different potencies, you cannot assume “4 mg of one equals 4 mg of the other.” A bigger number on a retatrutide vial does not make it a higher dose of the same thing.

The bigger reason the scales are not comparable is regulatory. Tirzepatide’s ladder is fixed in an FDA-approved prescribing label that has been studied across the SURMOUNT and SURPASS programmes. Retatrutide has only completed phase-2 testing — the doses studied (1, 4, 8 and 12 mg weekly in the obesity trial) were chosen to map the dose-response curve, not to define a finished product. Phase-3 trials are ongoing, so the eventual approved doses, titration steps, and even the top dose could all change or the drug may never be approved at all.

That uncertainty is exactly why we say it repeatedly: retatrutide is investigational. Treat any retatrutide figure here as background education on what researchers tested, never as a dose to copy.

Side-by-side: dosing at a glance

Property	Tirzepatide	Retatrutide
Regulatory status	FDA-approved (e.g. Zepbound, Mounjaro)	Investigational — not approved
Receptor targets	GIP + GLP-1 (dual)	GIP + GLP-1 + glucagon (triple)
Starting dose	2.5 mg / week (label)	~2 mg/week initial in phase-2 (trial only)
Dose steps studied	2.5, 5, 7.5, 10, 12.5, 15	1, 4, 8, 12 (phase-2 targets)
Max dose	15 mg / week (approved)	No approved max (12 mg studied)
Min step interval	4 weeks (label)	~4 weeks in trial protocols
Route	Subcutaneous, once weekly	Subcutaneous, once weekly (trials)

Tirzepatide figures reflect the FDA-approved weight-management label. Retatrutide figures are drawn from its published phase-2 obesity and type-2-diabetes trials and are not a prescribing schedule. Compounded vials are drawn manually, so you must calculate the draw volume yourself; always follow a prescriber’s instructions.

The titration timelines, visualised

Both molecules build up slowly to limit nausea, but they climb different ladders. Retatrutide’s trial schedule used larger jumps to reach high doses inside a research window; tirzepatide’s approved ladder takes smaller, fixed steps.

The dashed orange line is investigational. It is shown only to illustrate the shape of a research titration, not as a schedule anyone should follow.

How to turn any dose into syringe units

This is where most people get stuck, and the good news is the maths is identical for both drugs. The milligram dose is not what you read on the syringe — you read units, and a U-100 insulin syringe has 100 units per 1 mL. So the conversion is always: find the concentration (mg/mL), divide your dose by it to get mL, then multiply by 100 to get units.

Worked example 1 — tirzepatide (approved)

You have a vial reconstituted to 10 mg/mL and your prescribed dose is 5 mg/week.

5 mg ÷ 10 mg/mL = 0.5 mL. 0.5 mL × 100 units/mL = 50 units.

Draw 50 units on a U-100 syringe — half of a 1 mL syringe.

Worked example 2 — retatrutide (investigational, maths only)

For illustration only, take a 4 mg research dose from a vial at 10 mg/mL.

4 mg ÷ 10 mg/mL = 0.4 mL. 0.4 mL × 100 units/mL = 40 units.

Maths gives 40 units — but retatrutide is not approved; this is not a dose recommendation.

Worked example 3 — same dose, different concentration

The same 4 mg from a 20 mg/mL vial: 4 ÷ 20 = 0.2 mL × 100 = 20 units.

Double the concentration, half the units — for the identical milligram dose. This is why “how many units” has no single answer without the vial strength.

Dose-to-units reference chart

Common weekly milligram figures for each drug, shown as units on a U-100 syringe at two typical compounded concentrations. Retatrutide rows are trial doses, included only to show the arithmetic — not a schedule. Always confirm against your own vial label.

Dose	At 5 mg/mL	At 10 mg/mL
2.5 mg (tirz start)	50 units	25 units
5 mg (tirz)	100 units	50 units
7.5 mg (tirz)	— (over 1 mL)	75 units
15 mg (tirz max)	— (over 1 mL)	— (over 1 mL)
1 mg (reta trial)	20 units	10 units
4 mg (reta trial)	80 units	40 units
8 mg (reta trial)	— (over 1 mL)	80 units

Notice the “over 1 mL” cells: any draw above 1 mL exceeds a standard U-100 syringe, so higher doses need a stronger vial. If your calculated draw exceeds 1 mL, ask your pharmacy about a higher-concentration vial rather than splitting one dose into two injections.

How this is calculated

Every figure here uses two facts only: a U-100 syringe holds 100 units per mL, and concentration is dose-per-volume. There is no drug-specific magic constant — the arithmetic is identical for tirzepatide, retatrutide, or any other vialled peptide, which is the whole point of comparing them this way. The calculators on this site automate exactly this, and let you sanity-check the result against the chart above. None of this is medical advice, and the retatrutide examples are illustrative maths for an investigational compound only.

Frequently asked questions

Is retatrutide approved or available by prescription?

No. As of writing, retatrutide is investigational — it has completed phase-2 trials and is being studied in phase-3, but it is not approved by the FDA, MHRA, EMA, or any regulator. There is no official label, no approved dose, and no approved titration schedule. Tirzepatide, by contrast, is approved and prescribed.

Is retatrutide just a stronger tirzepatide?

No. Retatrutide activates three receptors (GIP, GLP-1 and glucagon) while tirzepatide activates two (GIP and GLP-1). They are different molecules on different milligram scales, so a higher number on a retatrutide vial does not mean a higher “dose” of the same drug.

Why does the chart only go to 12 mg for retatrutide?

Because 12 mg weekly was the highest dose tested in the published phase-2 obesity trial. It is not an approved maximum — the eventual approved doses, if the drug is ever approved, could be different. Treat 12 mg as a research data point, not a target.

Do I draw more units for retatrutide than tirzepatide?

Not necessarily. Units depend on vial concentration, not the drug name. A 4 mg dose from a 10 mg/mL vial is 40 units; a 5 mg tirzepatide dose from the same vial is 50 units. The stronger the vial, the fewer the units for the same milligrams.

Sources

This guide is for general educational purposes only and does not constitute medical advice. Always follow your prescriber’s specific instructions and consult a qualified clinician before changing any protocol.