"Retatrutide dosage chart" and "retatrutide dosage schedule" are two of the most-searched retatrutide questions in the United States right now, and "how much bac water for 30mg retatrutide" is one of the fastest-rising. People aren't asking what retatrutide is anymore. They're asking how to draw it up.
So here is the dosing piece, built only from what's documented: the trial schedules, the published side-effect table, the drug's half-life, and the plain arithmetic of vials, water and syringes. Where the internet's numbers have no source, we'll say so. Let's dive in.
TLDR
What is it? Retatrutide is Eli Lilly's investigational once-weekly GIP, GLP-1 and glucagon receptor agonist, with a half-life of about six days. For the full background, see our retatrutide guide.
Results: In the published Phase 2 trial, 48-week weight loss was 8.7% on 1 mg, 17.1% on 4 mg, 22.8% on 8 mg and 24.2% on 12 mg (Jastreboff et al., 2023, NEJM, PMID 37366315).
Dosage: Phase 2 started at 2 mg (or 4 mg) and stepped up every four weeks. Phase 3 reaches 4, 9 or 12 mg through a fixed 16-week escalation. No validated protocol exists outside a trial.
Safety: Starting at 2 mg instead of 4 mg cut nausea on the 8 mg dose from 60% to 17%. A reconstituted multi-dose vial should be thrown away 28 days after first use, which makes big vials wasteful at low doses.
Legal Status (October 2026): Not approved anywhere. Lilly plans to file with the FDA in Q1 2027. It cannot be compounded under US federal law.
Why Retatrutide Dosing Is the Question Now
US search interest in retatrutide has climbed by roughly three-quarters over the past year. "Retatrutide dose" searches nearly doubled over the same period, and "reconstitution calculator" is the rising query attached to "retatrutide dosage".
The reason is simple. The headline Phase 3 numbers landed this summer, the FDA filing slipped to the first quarter of 2027 (BioPharma Dive, July 2026), and a lot of people decided not to wait. Gray-market retatrutide is sold as freeze-dried powder, usually in 10, 20 or 30 mg vials, and turning that into a weekly dose is left entirely to the buyer.
The trials tell you which doses were tested and how fast to climb. Nothing tells you what's in the vial.
How Retatrutide Dosing Works
Two numbers explain almost everything about the schedule.
The first is the half-life: about six days (Urva et al., 2022, The Lancet, PMID 36354040). With weekly injections, a bit under half of each dose is still in your system when the next one goes in. Levels build up for roughly four weeks before they plateau, at around 1.8 times what a single dose produces. That's why the trials step up every four weeks. Each step gets a chance to fully land before the next one.
It also means a dose change takes weeks to show its full effect, up or down. If a new dose makes you sick on day two, it will likely be worse by day five before it gets better.
The second is that the side effects track the climb, not just the destination. In Phase 2, gastrointestinal side effects "occurred primarily during dose escalation" and were "partially mitigated" by a lower starting dose (Jastreboff et al., 2023). The 4-week ladder isn't paperwork. It's the main tolerability tool the trials used.
Retatrutide vs Tirzepatide vs Semaglutide: The Schedules Side by Side
Retatrutide has no label, so the closest comparison is the two approved weekly drugs and the trial schedules.
| Drug | Starting dose | How it climbs | Maintenance doses | Half-life |
|---|---|---|---|---|
| Semaglutide (Wegovy) | 0.25 mg weekly | Every 4 weeks: 0.5, 1, 1.7, 2.4 mg | 2.4 mg | About 1 week |
| Tirzepatide (Zepbound) | 2.5 mg weekly | +2.5 mg after at least 4 weeks | 5, 10 or 15 mg | About 5 days |
| Retatrutide, Phase 2 | 2 mg or 4 mg weekly | Every 4 weeks, reaching 12 mg by week 12 | 1, 4, 8 or 12 mg | About 6 days |
| Retatrutide, Phase 3 (TRIUMPH) | Not published in detail | Fixed escalation over 16 weeks | 4, 9 or 12 mg | About 6 days |
Sources: Wegovy and Zepbound US prescribing information; Jastreboff et al., 2023; Urva et al., 2022; Giblin et al., 2026, Diabetes, Obesity and Metabolism, PMID 41090431.
Notice the starting doses. Retatrutide's trial start of 2 mg sits close to tirzepatide's 2.5 mg, while semaglutide starts far lower. Online claims that you "need" to start at a fraction of a milligram aren't drawn from the obesity trials.
What the Trials Actually Showed About Dose
The Phase 2 obesity trial is the only published retatrutide trial that compared starting doses directly. Its side-effect table is the most useful dosing document that exists.
| Phase 2 arm | Start | Nausea | Vomiting | Fatigue | Stopped for side effects |
|---|---|---|---|---|---|
| Placebo | None | 11% | 1% | 4% | 0% |
| 1 mg | 1 mg | 14% | 3% | 4% | 7% |
| 4 mg | 2 mg | 18% | 12% | 12% | 6% |
| 4 mg | 4 mg | 36% | 12% | 6% | 9% |
| 8 mg | 2 mg | 17% | 6% | 3% | 14% |
| 8 mg | 4 mg | 60% | 26% | 9% | 6% |
| 12 mg | 2 mg | 45% | 19% | 10% | 16% |
Source: Jastreboff et al., 2023, NEJM, Table 3. Status: published.
The starting dose matters more than the final dose. At 8 mg, starting at 2 mg rather than 4 mg cut nausea from 60% to 17% and vomiting from 26% to 6%. The arms had only 33 to 69 people each, so the numbers bounce around, and the dropout column doesn't follow the same pattern. But the direction of the nausea finding is clear, and the authors drew the same conclusion.
Weight loss, meanwhile, was decided by where you ended up. The paper reports 48-week results by final dose, combining both starting doses: 8.7% on 1 mg, 17.1% on 4 mg, 22.8% on 8 mg, 24.2% on 12 mg, against 2.1% on placebo. The jump from 8 to 12 mg bought about a point and a half.
Phase 3 dropped the 8 mg dose for 9 mg and kept 4 and 12 mg. Every participant reaches their dose through a fixed escalation over 16 weeks, then holds for 64 weeks. Participants who can't tolerate a dose are allowed to step down and try climbing again, and to take a permanent dose reduction if that fails (Giblin et al., 2026). Lilly's Phase 3 toplines report up to 28.7% weight loss; none of the four obesity trials has been published.
Climb slowly from a low start, and stepping down is part of the protocol, not a failure of it. That's what the trials actually did.
Retatrutide Dosage: Trials vs What You'll Find Online
Let's be clear about what exists. There are trial schedules, which are documented. There is a large body of online dosing guides, which aren't.
The trial schedule
In Phase 2, the 12 mg group took 2 mg for four weeks, then 4 mg, then 8 mg, then 12 mg, reaching the top dose by week 12. Every escalation step was four weeks. Phase 3 uses a 16-week fixed escalation to 4, 9 or 12 mg, with the exact steps not published in the design paper.
What the online guides say
We looked at the dosing guides that rank for these searches. They disagree with each other badly. Suggested starting doses run from a quarter of a milligram to 4 mg a week, and "maintenance" ranges run from 1.5 mg to 12 mg. We found none that cites a clinical source for its numbers.
The only large dataset on actual community use is a preprint that analysed Reddit posts from 13,589 people reporting current retatrutide use. It isn't peer reviewed, and it measures what people wrote, not what was in their syringe (Sehgal et al., 2026, medRxiv). It doesn't establish a typical dose.
No validated retatrutide protocol exists outside a clinical trial. The trial schedule is a fact about the trial, not an instruction.
Reconstitution Math: How Much BAC Water for 10, 20 and 30 mg
There's no single right amount of water. More water means a weaker solution and more units per dose; less water means fewer units and less room for error at small doses. The concentration is simply the milligrams in the vial divided by the millilitres of bacteriostatic water you add.
Three common setups give round numbers: 10 mg in 2 mL, 20 mg in 2 mL or 30 mg in 3 mL, and 30 mg in 2 mL.
| Weekly dose | 5 mg/mL (10 mg vial + 2 mL) | 10 mg/mL (20 mg + 2 mL, or 30 mg + 3 mL) | 15 mg/mL (30 mg vial + 2 mL) |
|---|---|---|---|
| 1 mg | 20 units | 10 units | 6.7 units |
| 2 mg | 40 units | 20 units | 13.3 units |
| 4 mg | 80 units | 40 units | 26.7 units |
| 6 mg | 120 units (too much for one syringe) | 60 units | 40 units |
| 8 mg | 160 units (too much for one syringe) | 80 units | 53.3 units |
| 9 mg | 180 units (too much for one syringe) | 90 units | 60 units |
| 12 mg | 240 units (too much for one syringe) | 120 units (too much for one syringe) | 80 units |
Units are on a standard U-100 insulin syringe, where 100 units equals 1 mL. Check your own vial and water volume with the reconstitution calculator and the dosage calculator rather than trusting a table.
So: how much BAC water for a 30 mg vial? 3 mL gives 10 mg/mL, where every milligram is 10 units and the arithmetic is hard to get wrong. 2 mL gives 15 mg/mL, which keeps a 12 mg dose inside one syringe but turns low doses into fractions of a unit mark. At 10 mg/mL, a 0.3 mL syringe is easier to read accurately for the 1 to 2 mg doses at the start.
The 28-Day Rule and What It Does to Your Vial
This is the part most dosing charts leave out. FDA advises that a multi-dose vial of injectable medicine be used within 28 days of first use, because the risk of contamination rises after that. Discard it at 28 days "even if medication remains in the vial", whatever the seller's instructions say (FDA, concerns with unapproved GLP-1 drugs).
With weekly injections, that's four doses per vial. Here's what that means in practice.
| Weekly dose | Used in 28 days | 10 mg vial | 20 mg vial | 30 mg vial |
|---|---|---|---|---|
| 2 mg | 8 mg | 2 mg discarded | 12 mg discarded | 22 mg discarded |
| 4 mg | 16 mg | Runs out in week 3 | 4 mg discarded | 14 mg discarded |
| 8 mg | 32 mg | Runs out in week 2 | Runs out in week 3 | Runs out in week 4 |
| 12 mg | 48 mg | Not one full dose | Runs out in week 2 | Runs out in week 3 |
The pattern is simple: the vial that fits a 28-day window holds about four times your weekly dose. A 30 mg vial matches a weekly dose of about 7.5 mg. Starting a 30 mg vial at 2 mg means throwing away nearly three-quarters of it, or using it past the point FDA says is safe.
Two more handling notes from the same FDA guidance: use a new sterile needle and syringe for every dose, and don't use an injectable that arrives warm or without adequate cooling. Once mixed, keep the vial in the fridge.
Retatrutide Side Effects by Dose
The trial side effects are dose-related and concentrated during the climb. Nausea ran from 14% on 1 mg to 60% on 8 mg with a 4 mg start, against 11% on placebo, as the table above shows. Heart rate rose with dose, peaked around week 24 and then declined. The full safety record, including the skin-sensitivity signal and the Australian liver-injury alert, is in our retatrutide guide.
Does retatrutide make you tired?
In the trial, not much more than placebo. Fatigue was reported by 7% of everyone on retatrutide against 4% on placebo, and 10% on 12 mg (Jastreboff et al., 2023).
Online, it's a different picture. In the Reddit preprint, fatigue was one of the most commonly reported symptoms, alongside increased appetite, increased energy, nausea, food cravings, insomnia and a raised heart rate. That's a very different list from the trial's, which was dominated by gastrointestinal effects (Sehgal et al., 2026). Self-reports are noisy, but the gap is worth noticing. The trial measured the molecule; Reddit is measuring whatever was in the vials, at whatever doses people chose.
What about missed doses?
No missed-dose rule for retatrutide has been published. Approved weekly drugs in this class each have one in their label, and they differ: Zepbound allows a missed dose within 4 days, Wegovy if the next dose is more than 2 days away. Retatrutide has no label, and borrowing another drug's rule is a guess.
Is Retatrutide Legal?
Retatrutide is not approved anywhere. Lilly plans to file with the FDA in the first quarter of 2027, which puts a decision in late 2027 at the earliest. FDA has stated that retatrutide "cannot be used in compounding under federal law", so no pharmacy or telehealth clinic can lawfully supply it. Vials sold online as "research use only" are unapproved drugs from unregulated sources, and Lilly sued four such vendors in August. The details are in our guide and on the retatrutide sourcing page.
The Honest Take
The trials leave a clear dosing lesson, and it isn't a number. Start low, climb in four-week steps, and step down when it doesn't sit right. The published table shows a 2 mg start made the same 8 mg dose far easier to tolerate, and the 12 mg dose added little over 8 mg.
The arithmetic is the easy part. Ten milligrams per millilitre, ten units per milligram, four doses per vial before the 28-day line. Get those right and you have measured the powder accurately.
What the arithmetic can't tell you is whether the powder is retatrutide, how much of it there is, and what else is in there. Every number in the trial table was measured on Lilly's molecule. A dosing chart is only as good as the vial you apply it to.
FAQ
What is the retatrutide starting dose?
In the Phase 2 trial, 2 mg once weekly for most groups, with some groups starting at 4 mg. The 2 mg start produced much less nausea. Phase 3 hasn't published its exact starting step.
What does a retatrutide dosage schedule look like in the trials?
For the 12 mg group in Phase 2: 2 mg for four weeks, then 4 mg, then 8 mg, then 12 mg from week 12. Phase 3 reaches 4, 9 or 12 mg through a fixed 16-week escalation, then holds the dose.
How much BAC water for 30 mg retatrutide?
3 mL gives 10 mg/mL, so each milligram is 10 units on a U-100 syringe. 2 mL gives 15 mg/mL, so each milligram is about 6.7 units. Either works; 3 mL makes the math simpler. Check it with the reconstitution calculator.
How many units is 2 mg of retatrutide?
It depends on the concentration. At 10 mg/mL it's 20 units; at 5 mg/mL it's 40 units; at 15 mg/mL it's about 13 units.
How many doses are in a 10 mg vial?
Five doses at 2 mg, two and a half at 4 mg, one and a quarter at 8 mg. At 12 mg, a 10 mg vial doesn't hold one dose.
How long does reconstituted retatrutide last?
FDA advises discarding any multi-dose injectable vial 28 days after first use, even if some is left. Keep it refrigerated and use a new sterile needle every time. Nobody has published stability data for gray-market retatrutide.
What is the maximum dose of retatrutide?
12 mg once weekly is the highest dose tested in Phase 2 and Phase 3. In Phase 2 it added about 1.4 points of weight loss over 8 mg, with more side effects.
Does retatrutide make you tired?
Fatigue affected 7% of trial participants on retatrutide against 4% on placebo. It's reported far more often online, which may reflect dose, product quality, or eating much less.
Is retatrutide a GLP-1?
Partly. It activates the GLP-1 receptor, plus the GIP and glucagon receptors. That third receptor is what separates it from tirzepatide and semaglutide.
What if I miss a dose?
There's no published rule for retatrutide. Approved weekly drugs have label rules that differ from each other, and none of them applies to an unapproved compound.
Resources
Jastreboff et al. (2023), NEJM, Phase 2 obesity trial, dose arms and Table 3 adverse events — https://pubmed.ncbi.nlm.nih.gov/37366315/
Urva et al. (2022), The Lancet, Phase 1b, half-life of about 6 days — https://pubmed.ncbi.nlm.nih.gov/36354040/
Giblin et al. (2026), Diabetes, Obesity and Metabolism, TRIUMPH program design and escalation — https://pubmed.ncbi.nlm.nih.gov/41090431/
ClinicalTrials.gov, retatrutide Phase 2 arms (NCT04881760) — https://clinicaltrials.gov/study/NCT04881760
ClinicalTrials.gov, TRIUMPH-1 (NCT05929066) — https://clinicaltrials.gov/study/NCT05929066
Sehgal et al. (2026), medRxiv preprint, self-reported side effects among Reddit users — https://doi.org/10.64898/2026.05.28.26352819
FDA, concerns with unapproved GLP-1 drugs, including the 28-day vial guidance — https://www.fda.gov/drugs/postmarket-drug-safety-information-patients-and-providers/fdas-concerns-unapproved-glp-1-drugs-used-weight-loss
Zepbound (tirzepatide) US prescribing information — https://pi.lilly.com/us/zepbound-uspi.pdf
Wegovy (semaglutide) US prescribing information — https://www.novo-pi.com/wegovy.pdf
BioPharma Dive (July 2026), retatrutide FDA filing moved to Q1 2027 — https://www.biopharmadive.com/news/lilly-retatrutide-fda-application-obesity-drug-results/825987/
CNBC (July 23, 2026), Lilly Phase 3 results and filing plan — https://www.cnbc.com/2026/07/23/eli-lilly-will-file-for-approval-of-retatrutide-obesity-drug-in-2027.html
NPR (August 13, 2026), Lilly's lawsuits against retatrutide sellers — https://www.npr.org/2026/08/13/g-s1-138594/eli-lilly-files-6-lawsuits-against-businesses-selling-black-market-obesity-drug
