Retatrutide Dosage Guide: Trial Doses, Escalation Math and Volume Conversion

Retatrutide (LY3437943) is a single-molecule triple agonist of the GIP, GLP-1, and glucagon receptors, and it produced the largest weight reductions ever documented for a non-surgical intervention in its Phase 2 program. But the numbers that get quoted (up to -24.2% body weight) are attached to specific weekly doses studied in specific trials. This guide breaks down exactly what each of the four studied dose arms delivered, explains the units that trip most researchers up, and walks through the arithmetic of converting a target milligram dose into an injection volume at any reconstitution concentration. It closes with an "efficacy per mg" analysis that most competing write-ups skip entirely.

Research-use framing only. Retatrutide is an investigational compound. It is not approved by the FDA or any regulatory agency for human use, and everything below describes what the published clinical literature documents, not a recommendation to dose. The dose arms discussed were administered under trial supervision. Nothing here is medical advice.

The Four Dose Arms That Were Actually Studied

Every credible retatrutide efficacy figure traces back to two Phase 2 trials: the obesity trial (Jastreboff et al., New England Journal of Medicine, 2023) and the type 2 diabetes trial (Rosenstock et al., The Lancet, 2023). The obesity trial (n=338, 48 weeks) tested four weekly subcutaneous maintenance doses: 1 mg, 4 mg, 8 mg, and 12 mg. There is no published human efficacy data for doses outside this range. Anything a vendor claims about a 15 mg or 20 mg "arm" is fabricated.

Here is the headline efficacy table, with each dose mapped to its documented 48-week outcome.

A few figures worth anchoring to memory, because they are the ones that are real:

• At 24 weeks, the 12 mg arm was only around -17.5%. The curve had not plateaued at the end of the trial, meaning the 48-week -24.2% was still climbing.
• 100% of both the 8 mg and 12 mg arms reached at least 5% weight loss at 48 weeks. Not "most." All of them.
• Roughly 26% of the 12 mg arm reached at least 30% weight loss, a threshold no GLP-1 monotherapy approaches.

For the glycemic side, the type 2 diabetes trial (n=281, 36 weeks) is the relevant source. It showed HbA1c reductions up to -2.02% at 12 mg, versus -0.01% for placebo and -1.41% for the dulaglutide 1.5 mg active comparator, with weight loss up to -16.94% at 12 mg over the shorter 36-week window. A separate liver fat substudy reported liver fat content reduced by up to ~86% at 24 weeks in participants with MASLD, with about 80% reaching normal liver fat content (under 5%).

Units: mg vs mcg vs "Syringe Units"

The single most common dosing error in this space is a unit-confusion error, so start here before touching any math.

The critical trap: "units" on a U-100 insulin syringe are not a dose of drug. They are a measure of liquid volume. On a U-100 syringe the barrel is graduated so that 100 units equals exactly 1 mL, and each single unit mark equals 0.01 mL. The syringe has no idea how much peptide is dissolved in that liquid. That depends entirely on how the vial was reconstituted. A "10 unit" draw could contain 1 mg or 5 mg of retatrutide depending on concentration. This is why you can never copy someone else's "units" number without knowing their concentration.

Converting a Target mg Dose Into Injection Volume

There is exactly one formula, and it never changes:

Volume to inject (mL) = target dose (mg) divided by concentration (mg per mL)

To get syringe units on a U-100 syringe, multiply the mL result by 100. Concentration is set at reconstitution: concentration (mg/mL) = total mg of peptide in the vial divided by mL of bacteriostatic water added. Our reconstitution calculator does all of this automatically, but you should understand the arithmetic so you can catch when a number looks wrong.

Worked Example 1: A 10 mg vial, reconstituted with 2 mL, targeting a 4 mg dose

• Concentration = 10 mg / 2 mL = 5 mg/mL
• Volume = 4 mg / 5 mg/mL = 0.8 mL
• Syringe units = 0.8 mL x 100 = 80 units on a U-100 syringe

Worked Example 2: Same 10 mg vial, but reconstituted with 1 mL, targeting the same 4 mg dose

• Concentration = 10 mg / 1 mL = 10 mg/mL
• Volume = 4 mg / 10 mg/mL = 0.4 mL
• Syringe units = 0.4 mL x 100 = 40 units

Note what just happened: the exact same 4 mg dose is now 40 units instead of 80, purely because the vial was reconstituted with half the water. This is the entire reason "how many units do I draw" is an unanswerable question without the concentration.

Worked Example 3: A 5 mg vial, reconstituted with 1 mL, targeting the 1 mg starting dose

• Concentration = 5 mg / 1 mL = 5 mg/mL
• Volume = 1 mg / 5 mg/mL = 0.2 mL
• Syringe units = 0.2 mL x 100 = 20 units

Worked Example 4: A 15 mg vial, reconstituted with 1.5 mL, targeting an 8 mg dose

• Concentration = 15 mg / 1.5 mL = 10 mg/mL
• Volume = 8 mg / 10 mg/mL = 0.8 mL
• Syringe units = 0.8 mL x 100 = 80 units

A convenient reference table for the four studied doses at three common concentrations, expressed in U-100 syringe units:

Notice the practical constraint: a standard 1 mL U-100 syringe holds only 100 units. At 5 mg/mL, an 8 mg or 12 mg dose exceeds one syringe, which is itself a signal that a low concentration is impractical for the top doses. Higher concentrations keep the draw inside a single syringe but make each unit mark "worth" more drug, so misreading by a couple of units costs more.

Timing and Consistency

Retatrutide has a terminal half-life of about 6 days, with a Tmax (peak concentration) roughly 24 to 48 hours after injection. That long half-life is what supports the once-weekly dosing used across the entire Phase 2 program.

Three practical consequences fall directly out of that pharmacokinetics:

• Pick one day of the week and keep it. Weekly cadence matters far more than clock precision. A dose given Monday morning versus Monday night is a trivial difference against a 6-day half-life.
• Time of day is essentially irrelevant. With a multi-day half-life, plasma levels barely move over a single 24-hour window, so morning versus evening does not meaningfully change exposure. Some researchers dose in the evening simply so that peak nausea (which tends to arrive 24 to 48 hours later) overlaps with sleep.
• A missed dose is not an emergency. Because roughly half the compound is still present after 6 days, missing a scheduled injection by a day or two produces only a modest dip. The general logic documented for long-half-life weekly incretins is: if the miss is caught within a couple of days, dose when remembered and then resume the normal weekly day; if it has been most of a week, take the next dose on the regular schedule rather than doubling up. Doubling doses is exactly what drives the dose-dependent gastrointestinal spikes described in the side effects guide. Logging each injection in a tracker removes the guesswork about when the last dose actually landed.

Dose Selection Reasoning: Match the Dose to the Objective

One of the more overlooked findings in the retatrutide data is that glycemic endpoints and maximal weight endpoints do not peak at the same dose economics. The glycemic response saturates earlier on the dose curve than the weight response does.

In the type 2 diabetes trial, HbA1c reduction was already deep at mid-range doses, and the marginal HbA1c benefit from pushing to 12 mg was modest relative to the jump seen at lower steps. Weight loss, by contrast, kept climbing across the full dose range and had not fully plateaued even at 12 mg by 48 weeks. This split matters for anyone reasoning about research objectives:

The takeaway is not "always go to 12 mg." It is that the "best" dose depends on which endpoint you are actually reasoning about, and the two most-studied endpoints do not share the same optimal point on the curve.

The Information-Gain Section: Efficacy Per mg

Here is the analysis almost no competing article runs. Instead of just listing the weight-loss numbers, look at the marginal weight loss each dose step buys you, using the 48-week obesity figures.

Read the right-hand column top to bottom and the shape of the drug becomes obvious: this is a classic saturating dose-response curve with sharply diminishing returns. The first milligram does enormous work. The 1-to-4 step still buys a large 8.4 points. The 4-to-8 step buys a respectable 5.7 points. But the final 8-to-12 step, despite adding the same 4 mg of drug as the 4-to-8 step, buys only 1.4 additional points of weight loss.

What that curve implies:

• The 8-to-12 step delivers roughly one quarter the marginal weight loss of the 4-to-8 step, for the identical 4 mg increase in dose. On a pure efficacy-per-mg basis, it is the least productive step in the entire range.
• Because gastrointestinal adverse events are dose-dependent and worst during escalation, the top step trades a large tolerability cost against a small efficacy gain. The 12 mg arm carried the heaviest GI burden and the higher end of the roughly 6 to 16% discontinuation range seen across arms.
• One caveat keeps 12 mg interesting: the 12 mg curve had not plateaued at 48 weeks. Its per-mg return looks poor at the 48-week snapshot, but a longer trial might have widened the 12 mg advantage. The saturating shape is a 48-week observation, not necessarily the final steady state.

This is precisely the kind of trade-off math the efficacy and benefits review and the broader triple-agonist research overview exist to contextualize. If you want to see how the curve compares against tirzepatide and semaglutide, those compounds saturate at lower absolute weight-loss ceilings.

The Three Most Common Dosing-Arithmetic Mistakes

Nearly every real dosing error reduces to one of three arithmetic mistakes. Each one is avoidable with a single sanity check.

Mistake 1: Confusing mg with units

Someone reads "the 4 mg dose" and draws 4 units on the syringe, or reads "80 units" and believes they injected 80 mg. These are different kinds of quantity entirely: mg is drug mass, units are liquid volume. At a typical 10 mg/mL concentration, a true 4 mg dose is 40 units, so drawing "4 units" delivers one-tenth of the intended dose. The check: mg is a property of the drug, units are a property of the syringe, and you always need the concentration to move between them.

Mistake 2: Misreading a U-100 syringe

U-100 syringes are marked so that 100 units = 1 mL and each small tick is 1 unit = 0.01 mL. Errors happen when someone counts the big numbered gradations (often marked every 10 units) as single units, or lines up the wrong edge of the plunger stopper. Being off by "one line" you assume is 1 unit but is actually 10 units is a tenfold error. The check: confirm that the full barrel reads 100, and that ten small ticks sit between each large number.

Mistake 3: Reusing an old volume after re-reconstituting at a different concentration

This is the most insidious one because the arithmetic was correct the first time. You reconstituted a vial, calculated that your dose was 40 units, and dosed correctly for weeks. Then a new vial arrives, or you add a different volume of bacteriostatic water, changing the concentration. If you keep drawing "40 units" out of habit, you are now delivering a completely different milligram dose. Recall Worked Examples 1 and 2: the same 4 mg dose was 80 units at 5 mg/mL but 40 units at 10 mg/mL. The check: every time a new vial is reconstituted, recompute the volume from scratch rather than reusing the last unit count. Run the calculator again for each new vial.

Where This Sits in the Broader Research Picture

Retatrutide's Phase 3 program, called TRIUMPH (TRIUMPH-1 through TRIUMPH-4 plus a cardiovascular outcomes study), was ongoing as of this writing, and it is the source that will eventually confirm or revise these Phase 2 dose-response numbers at scale. Until then, the 1/4/8/12 mg arms above are the entire evidence base for retatrutide dosing, and any figure outside them is not data.

A note on grey-market marketing: retatrutide is frequently sold under the nickname "GLP-3" or "triple G." That is marketing shorthand, not pharmacology. There is no such thing as a GLP-3 receptor. The compound is a triple agonist of three real receptors (GIP, GLP-1, and glucagon), and "GLP-3" is simply a made-up label. If a listing leans on that term, treat it as a signal to scrutinize the vendor more carefully. New to the space? The learn hub and the knowledge quiz are the fastest ways to build the background, and sourcing questions belong in the where-to-buy overview.

Frequently Asked Questions

What were the exact retatrutide doses used in the trials?

The Phase 2 obesity trial tested four weekly subcutaneous maintenance doses: 1 mg, 4 mg, 8 mg, and 12 mg, over 48 weeks in 338 adults with obesity. The type 2 diabetes trial used a comparable range over 36 weeks. There is no published human efficacy data for any dose outside this range.

How do I convert a milligram dose into syringe units?

Divide your target dose in mg by the vial concentration in mg/mL to get the volume in mL, then multiply by 100 to get units on a U-100 syringe. For example, a 4 mg dose from a vial reconstituted to 10 mg/mL is 0.4 mL, which is 40 units. The calculator does this automatically, and you should recompute it every time you reconstitute a new vial.

Does the time of day I dose matter?

With a terminal half-life of about 6 days, plasma levels barely move over a single 24-hour window, so morning versus evening makes no meaningful difference to exposure. Consistency of the weekly day matters far more than the clock. Some researchers dose in the evening so that peak nausea, which tends to arrive 24 to 48 hours later, overlaps with sleep.

What should happen if a dose is missed?

Because roughly half the compound remains after 6 days, a missed dose is not an emergency. The general logic for long-half-life weekly incretins is to dose when remembered if the miss is caught within a day or two, then resume the normal weekly day; if it has been most of a week, take the next dose on the regular schedule rather than doubling up. Doubling doses drives the dose-dependent gastrointestinal side effects.

Why does the 12 mg dose deliver so little extra weight loss over 8 mg?

Retatrutide follows a saturating dose-response curve. The 8-to-12 mg step buys only about 1.4 additional percentage points of weight loss over 48 weeks, roughly one quarter of what the 4-to-8 mg step delivered for the same 4 mg increase. That said, the 12 mg curve had not plateaued at 48 weeks, so a longer trial might have widened its advantage.

Is "GLP-3" a real receptor?

No. "GLP-3" and "triple G" are grey-market marketing nicknames, not pharmacology. Retatrutide is a triple agonist of the GIP, GLP-1, and glucagon receptors, all of which are real and well-characterized. There is no GLP-3 receptor.

Which dose is best for glycemic versus weight-loss objectives?

The glycemic response (HbA1c) saturates earlier on the dose curve, so lower doses capture most of that effect, while maximal weight loss keeps climbing across the full range and had not plateaued at 12 mg by 48 weeks. The two most-studied endpoints therefore do not share the same optimal dose, which is why the objective should drive the reasoning rather than a single default number.

Key Research Citations