Retatrutide 20mg Dosage Chart: IU Per Dose at 2mL or 4mL

Retatrutide 20mg Dosage Chart: Units Per Dose

Find your dose on the left, then read across to the mix you used. Searches often call these numbers IU. On an insulin syringe they are volume marks: U-100 means 100 units per mL (the strength printed on Lilly's Humulin N label), so 1 unit is 0.01mL of liquid, not a unit of retatrutide.

Amber cells are more than 100 units, past the capacity of a 1mL (100-unit) U-100 syringe. The 1mg row was a Phase 2 dose arm; 9mg is a dose from the Phase 3 TRIUMPH-1 trial.

How to Read It

Every number assumes the vial holds exactly 20mg and that the powder is fully dissolved in exactly the stated volume of water. The 20mg part is only an assumption: FDA describes retatrutide products falsely labeled for research purposes as of unknown quality (see the notice below). The table converts a dose you already have into a syringe reading. It doesn't tell anyone which dose to take.

Which Mix for a 20mg Vial: 2mL or 4mL?

Both mixes are commonly used for this vial size, and neither is more correct. They hold the same 20mg. What changes is how much drug sits behind each syringe mark, shown below for every whole-mL volume from 1 to 5mL.

Values in units on a U-100 syringe. Shaded rows are the two common mixes. Amber = over 100 units.

• 2mL (10mg/mL). Each unit carries 0.1mg. Every dose up to 9mg fits a 1mL syringe; 12mg (120 units) is the one that doesn't. The starting draw is small, 20 units for 2mg.
• 4mL (5mg/mL). Half the drug per unit (0.05mg). The 2mg start is 40 units, a bigger mark, and misreading by one unit moves the dose half as far in mg. The cost is that 8mg and 12mg both run past 100 units.
• 1mL (20mg/mL). All four steps fit (12mg is 60 units), but the 2mg start drops to 10 units and each unit carries 0.2mg.
• 3mL and 5mL. Both give whole-number unit counts. At 5mL, 4mg is exactly 100 units, a full barrel, and nothing above it fits.

If the 2mL column looks familiar, that's because 20mg in 2mL and 30mg in 3mL are the same 10mg/mL strength. The units match the 30mg vial dosage chart exactly; only how long the vial lasts is different.

This page assumes the vial is already mixed. For the mixing itself, see the step-by-step reconstitution guide. The full reconstitution chart crosses every vial size with every water volume.

The Phase 2 Ladder on the Retatrutide 20mg Dosage Chart: 2, 4, 8, 12mg

The doses in the chart come from the Phase 2 obesity trial (Jastreboff et al., NEJM 2023; NCT04881760). Adults were randomized to once-weekly retatrutide at 1, 4, 8 or 12mg, or placebo, for 48 weeks. The 12mg group started at 2mg and climbed to 4mg, then 8mg, then 12mg. The 1mg group started at 1mg. The 4mg and 8mg groups each ran in two versions, one starting at 2mg and one at 4mg. The paper's abstract reports that the lower 2mg start partly reduced the gastrointestinal side effects.

The trial protocol, posted with the ClinicalTrials.gov record, raises the dose in 4-week increments until the maintenance dose is reached. For the 12mg group that means 2mg from Week 0, 4mg from Week 4, 8mg from Week 8 and 12mg from Week 12, which matches the registry's note that escalation ran up to Week 12. The trial counts the first injection as Week 0. This page counts it as week 1, so the vial map shows the first 12mg dose in week 13: same timing, counted differently.

TRIUMPH-1 used its own escalation, and this page doesn't map it; the vial map below follows the Phase 2 ladder only. Not every Phase 2 group went to 12mg, either. The 4mg and 8mg groups stopped climbing at those doses.

How Many Doses Does a 20mg Retatrutide Vial Give?

Two limits apply, and whichever hits first decides. One is volume: 20mg divided by the dose, rounded down. The other is time. FDA generally recommends using multi-dose vials of sterile medications within 28 days of first use, and for compounded GLP-1 vials its page on unapproved GLP-1 drugs tells consumers: "Discard the vial within 28 days after the first use, even if medication remains in the vial." Weekly doses on days 0, 7, 14 and 21 fit inside that window. A fifth would land on day 28 itself, so this page counts four. FDA's wording (within 28 days after first use) doesn't say whether a draw on day 28 itself counts. If it does, five weekly draws fit. The 1mg and 2mg rows then rise to 5 draws (15mg and 10mg unused), the 4mg row becomes a tie with 0mg unused, the crossover moves to 4mg, and the vial map below needs 7 vials instead of 8.

Assumes a vial that holds its labeled 20mg, no solution lost in the vial or needle, and each dose drawn from one vial.

The crossover sits at 5mg, because 20mg split four ways is 5mg. Below 5mg a week, the 28-day clock ends the vial before the drug runs out. At exactly 5mg, both limits arrive together on day 21. Above 5mg, the drug runs out first. The water volume changes none of this: 20mg is 20mg whether it sits in 2mL or 4mL.

One caveat on the 28-day figure: FDA gives it as general advice for multi-dose vials of sterile medication, on a page about compounded GLP-1 vials, and adds that a vial shouldn't be used past 28 days even if the compounder's instructions allow longer. It is not retatrutide-specific stability data.

Mapping the Phase 2 Ladder Onto 20mg Vials

Put the two limits together and you can count vials. The map follows the Phase 2 12mg group's ladder (2, 4, 8, then 12mg) across its first 16 weeks, using the protocol's 4-week escalation steps. It also assumes one vial open at a time (a new vial is opened only when the current one can't supply the next dose, because too little is left or it has reached the 28-day mark), a 28-day discard after first use, and each dose drawn from a single vial.

Eight 20mg vials over 16 weeks, with one vial open at a time. Indigo is the mg injected from each vial, gray the mg left unused. Steps follow the Phase 2 protocol's 4-week increments.

On this arithmetic, 16 weeks inject 104mg (8 + 16 + 32 + 48) from 160mg of vials, leaving 56mg unused. Most of that leftover, 32mg, comes from the four 12mg weeks, since a 20mg vial holds one 12mg dose and 8mg stays behind each time. The biggest single leftover is the first vial, though: four 2mg doses use 8mg, and 12mg is still in it at the 28-day mark.

Keep one vial open at a time but set aside the 28-day advice, and the same ladder fits in 7 vials by volume alone. On this page's four-dose count, the 28-day discard is what adds the eighth.

Assumptions: each vial holds its labeled 20mg, no solution is lost in the vial or needle, one vial is open at a time and each dose comes from it, each escalation step lasts 4 weeks (per the Phase 2 protocol), the map stops after four weeks at 12mg (in the trial, the 12mg group stayed on 12mg through the end of the 48-week treatment period), and vials are discarded 28 days after first use. The count changes if more than one vial is in use at once. This is arithmetic for a hypothetical correctly labeled vial, not a dosing or buying recommendation.

Syringe Capacity and Reading the Draw

A 1mL U-100 syringe holds 100 units. At the 2mL mix, every dose through 9mg fits (9mg is 90 units) and 12mg needs 120. At the 4mL mix, 2mg and 4mg fit, while 8mg (160), 9mg (180) and 12mg (240) don't.

This page won't say how to give a dose that's over 100 units. That's a question for a clinician, and retatrutide has no approved label or instructions for use to answer it.

Check the spacing of the marks on your own syringe before you draw. On a barrel marked every 2 units, an odd count falls between two lines; 1mg at a 3mL mix, for example, is 15 units. For a vial, water and dose combination not shown here, the retatrutide units calculator does the same arithmetic.

Investigational Drug and FDA Notice

References

1. Jastreboff AM, Kaplan LM, Frías JP, et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity: A Phase 2 Trial. New England Journal of Medicine. 2023;389(6):514-526. DOI: 10.1056/NEJMoa2301972. PMID: 37366315. (4mg and 8mg arms with 2mg or 4mg starts, a 12mg arm with a 2mg start, and a 1mg arm without escalation, over 48 weeks; gastrointestinal events partly reduced with the 2mg start.)
2. ClinicalTrials.gov. A Study of LY3437943 in Participants Who Have Obesity or Are Overweight. NCT04881760. Arm descriptions and results. clinicaltrials.gov/study/NCT04881760. Study Protocol J1I-MC-GZBF(b), approved 29 August 2022, section 4.1.1.1: Prot_000.pdf. (12mg arm sequence of 2, 4, 8 and 12mg once weekly; dose increased in 4-week increments; escalation up to Week 12.)
3. Jastreboff AM, Kaplan LM, Davies MJ, et al. Retatrutide, a Triple Hormone Receptor Agonist, for Treatment of Obesity. New England Journal of Medicine. 2026 (online ahead of print). DOI: 10.1056/NEJMoa2604169. PMID: 42814954. (TRIUMPH-1 Phase 3 doses of 4, 9 and 12mg for 80 weeks.)
4. U.S. Food and Drug Administration. FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss. Content current as of October 1, 2026. fda.gov. (28-day multi-dose vial discard; retatrutide cannot be used in compounding; warnings on products falsely labeled for research purposes.)
5. Eli Lilly and Company. HUMULIN N (insulin isophane human) injectable suspension, Prescribing Information, section 3. DailyMed set ID f6edd793-440b-40c2-96b5-c16133b7a921, effective December 2025. dailymed.nlm.nih.gov. (Cited only for the U-100 definition: 100 units per mL.)
6. ClinicalTrials.gov. Search results for retatrutide (34 records, searched October 2026). clinicaltrials.gov/search?term=retatrutide. (No arm description returned names a dose above 12mg.)

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