Peptide library · GLP-1 class
Retatrutide dosage, titration & reconstitution guide
Every dose, escalation step and weight-loss figure below comes from the published Phase 2 trials of retatrutide (LY3437943) — plus the reconstitution arithmetic people actually get wrong.
Researched by DoseRoutine Research TeamReviewed for accuracy by Nicholas Alexander, RSE, SO, PMPLast updated Educational reference only — not medical advice. Always confirm dosing and safety decisions with a licensed clinician.
7 peer-reviewed sources
- Triple agonist — GLP-1 + GIP + glucagon, the third receptor being what separates it from tirzepatide.
- Trial doses were 1, 4, 8 and 12 mg weekly, all reached by slow escalation over months.
- Mean weight reduction at 48 weeks reached about 24% on 12 mg — and the curve had not flattened.
- Concentration (mg/mL) = vial mg ÷ mL of BAC water. Units to draw = mL × 100.
- Not approved anywhere. Grey-market vials are unverified for identity, dose and sterility.
Retatrutide has no marketing authorization anywhere. Every figure on this page comes from published clinical trials run with medical supervision, screening and monitoring. This is educational reference material so you can understand the research — not a protocol to self-administer.
What retatrutide is
Retatrutide (LY3437943) is a single peptide that activates three receptors at once: GLP-1, GIP and glucagon. GLP-1 and GIP suppress appetite and slow gastric emptying; the glucagon arm is the novel part, increasing hepatic energy expenditure and fat oxidation. That third lever is why the Phase 2 results outran the dual agonists.
Appetite suppression, slower gastric emptying, improved insulin response. The mechanism semaglutide relies on alone.
Adds insulin sensitivity and appears to soften nausea relative to GLP-1 alone. The receptor tirzepatide bolted on.
Raises energy expenditure and hepatic fat oxidation — extra output, and also the source of raised heart rate and fasting glucose.
Half-life supports once-weekly dosing. The trade-off of the third receptor is that glucagon agonism can push fasting glucose and heart rate up, which is manageable inside a monitored trial and much less so unsupervised.
What the Phase 2 trials actually showed
338 adults with obesity were randomized to placebo or one of the retatrutide arms for 48 weeks. Mean weight reduction from baseline, by arm:
Bars show mean weight reduction at 48 weeks. At 24 weeks the same arms sat at Placebo 1.6%, 1 mg 7.2%, 4 mg 12.9%, 8 mg 17.3%, 12 mg 17.5%. Source: NEJM 2023.
Two things matter more than the headline number. First, the weight curve had not plateaued at 48 weeks, so the true ceiling is unknown. Second, these are means — individual responses spread widely in both directions, and the trial population was screened, supervised and supported.
Retatrutide dosage in the Phase 2 trials
The obesity trial randomized participants to 1, 4, 8 or 12 mg weekly subcutaneous injections, all reached by slow escalation. The escalation pattern below reflects the published schedule.
| Step | Weekly dose | Notes |
|---|---|---|
| Weeks 1–4 | 1–2 mg weekly | Starting step; most nausea shows up here |
| Weeks 5–8 | 2–4 mg weekly | Only if the prior step was tolerated |
| Weeks 9–16 | 4–8 mg weekly | Escalation held or reversed on GI symptoms |
| Week 17+ | 8–12 mg weekly | Highest trial arms; largest side-effect burden |
The single most important detail is the pace, not the ceiling. Gastrointestinal tolerability tracks how fast you escalate. Trial protocols held or stepped back down whenever nausea or vomiting appeared, and there was no benefit to rushing.
What the arc looks like month by month
Starting step. Appetite suppression is usually noticeable within the first one or two injections; nausea is at its most likely here. Weight change is small and largely water. Establish the protein target and training schedule now, not later.
First escalations. Each step up restarts the gastrointestinal adjustment for roughly a week. Fat loss becomes clearly visible on a weekly average, and food volume drops enough that protein and fiber need deliberate planning.
Higher steps. Trial participants were around 13–17% below baseline by week 24. Strength and energy dips here almost always trace back to under-eating protein or not training rather than the compound itself.
The curve had still not plateaued at 48 weeks in trials, with the top arms reaching about 24%. This is also where gallbladder issues, loose skin and lean-mass questions become the practical concerns rather than nausea.
Reconstitution math
Lyophilized vials need bacteriostatic water before anything can be measured. The whole calculation is one division: concentration (mg/mL) = vial strength (mg) ÷ BAC water added (mL). Then volume to draw (mL) = dose (mg) ÷ concentration, and a U-100 insulin syringe has 100 units per mL.
Plunger position for 80 units on a U-100 insulin syringe.
| Vial | BAC water | Concentration | Example draw |
|---|---|---|---|
| 5 mg | 1 mL | 5 mg/mL | 2 mg = 0.4 mL = 40 units |
| 10 mg | 2 mL | 5 mg/mL | 4 mg = 0.8 mL = 80 units |
| 10 mg | 1 mL | 10 mg/mL | 4 mg = 0.4 mL = 40 units |
| 20 mg | 2 mL | 10 mg/mL | 8 mg = 0.8 mL = 80 units |
Step by step
- Work out the concentration first. Divide vial strength in mg by the milliliters of bacteriostatic water you intend to add. A 10 mg vial with 2 mL gives 5 mg/mL. Decide this before the needle goes anywhere near the vial.
- Clean both stoppers. Swab the rubber stopper of the peptide vial and the bacteriostatic water vial with alcohol and let them air dry. Wash hands and work on a clean, uncluttered surface.
- Draw the diluent. Draw the planned volume of bacteriostatic water into a syringe, checking the plunger against the barrel markings at eye level.
- Add it down the vial wall. Insert the needle at an angle and let the water run slowly down the inside wall of the vial. Never squirt it directly onto the powder cake — force damages the peptide.
- Swirl, never shake. Roll or swirl gently until the solution is completely clear. Shaking creates foam and can denature the peptide. Do not use it if it stays cloudy or shows particles.
- Label and refrigerate. Write the concentration and the date on the vial, store it at 2–8 °C, and respect the beyond-use date of the bacteriostatic water (typically about 28 days).
- Convert the dose to syringe units. Volume in mL equals dose in mg divided by concentration in mg/mL; a U-100 insulin syringe has 100 units per mL, so multiply the milliliters by 100 to get units.
Double-check the decimal place every single time. A 10× error on a milligram dose is the failure mode that actually hurts people.
Enter vial strength, diluent volume and your target dose and get the exact insulin syringe units to draw — no mental math at the kitchen counter.
Open the peptide dosage calculatorRetatrutide vs tirzepatide vs semaglutide
| Compound | Receptors | Trial weight loss | Status |
|---|---|---|---|
| Retatrutide | GLP-1 + GIP + glucagon | ~24% at 48 weeks (12 mg) | Investigational |
| Tirzepatide | GLP-1 + GIP | ~21% at 72 weeks (15 mg) | Approved |
| Semaglutide | GLP-1 | ~15% at 68 weeks (2.4 mg) | Approved |
These trials were never run head to head, so treat the percentages as indications of scale rather than a ranking. The meaningful difference is regulatory: two of the three can be prescribed, monitored and sourced from a pharmacy. One cannot.
Deeper comparison: semaglutide vs tirzepatide.
Side effects and how they were managed
| Effect | When it shows up | Practical handling |
|---|---|---|
| Nausea | Peaks in the 1–2 weeks after each step up | Smaller meals, less fat, slow escalation, hold the step rather than pushing through |
| Vomiting / diarrhea | Escalation phase, dose-dependent | Aggressive hydration and electrolytes; persistent vomiting is a stop-and-call-a-doctor sign |
| Constipation | Anytime, worse with low food volume | Fiber, fluid, movement; low intake is usually the real cause |
| Raised heart rate | Dose-dependent across trial arms | Track resting HR weekly; sustained double-digit rises warrant medical review |
| Higher fasting glucose | Highest doses, via glucagon agonism | Fasting glucose and HbA1c monitoring; a known trade-off of the third receptor |
| Lean-mass loss | Throughout rapid weight loss | Protein target, resistance training, body composition rather than scale weight |
| Gallbladder symptoms | Secondary to fast weight loss | Right-upper-abdominal pain after fatty meals needs assessment, not patience |
Gastric emptying also slows, which changes absorption of oral medications — including oral contraceptives, thyroid medication and anything with a narrow absorption window.
Protecting muscle while the weight comes off
Losing 20%+ of body weight without losing meaningful lean mass is not automatic. Across GLP-1-class trials a substantial share of total weight lost has been fat-free mass — part of it water and glycogen, part of it muscle. Three levers change the ratio:
- Protein first. Roughly 1.6 g per kg of target body weight per day, front-loaded into meals, because total food volume is falling fast.
- Resistance training 2–4× a week. The stimulus that tells the body which tissue to keep. Cardio does not do this job.
- Escalate no faster than you can eat. If a dose step means you cannot hit protein, that step was too early.
Measure it: scale weight alone will happily hide a bad body-composition trend. Grip strength and your working sets are a free proxy. Muscle-support compounds are a distant third priority behind protein and training.
Monitoring checklist
- Resting heart rate, weekly, same conditions each time
- Fasting glucose and HbA1c at baseline and every 3 months
- Lipid panel, liver and kidney function at baseline and periodically
- Body composition or at minimum grip strength and key lifts, monthly
- Blood pressure, hydration and bowel habit during escalation
- Protein intake and training frequency — the two variables you fully control
Who should avoid it entirely
- Personal or family history of medullary thyroid carcinoma or MEN2
- Previous pancreatitis
- Severe gastroparesis or significant gastrointestinal disease
- Active gallbladder disease
- Pregnancy, breastfeeding or planning pregnancy
- Type 1 diabetes, or insulin/sulfonylurea use without prescriber supervision
- Tested athletes — GLP-1-class agents are prohibited in sport
GLP-1-class compounds interact with insulin and sulfonylureas, oral contraceptives, thyroid medication and anything with a narrow absorption window. Run your full routine before adding one.
Open the interaction checkerStopping, and what happens after
No withdrawal data exists for retatrutide yet, but the class pattern is consistent. In SURMOUNT-4, participants who had lost weight on tirzepatide and then switched to placebo regained roughly 14% of body weight over the next year, while those who continued kept losing. Appetite regulation reverts when the signal stops.
The practical implication is that the habits built during the losing phase — protein intake, training, sleep, meal structure — are the part that persists. Treating the compound as the whole plan sets up the regain.
Sourcing, purity and the legal picture
Retatrutide has no approved product, so there is no pharmacy version. Everything on the market is sold as a research chemical, and "for research use only" is a liability shield rather than a quality claim. Independent testing of grey-market peptides has repeatedly turned up:
- Vials under- or over-dosed relative to the printed strength
- Wrong or partially degraded peptide sequences
- Endotoxin and bacterial contamination from non-sterile filling
- Residual solvents and unidentified process impurities
A third-party certificate of analysis matched to the specific batch number, ideally with HPLC purity and mass-spec identity, is the minimum evidence — and it still says nothing about sterility. It is also prohibited in tested sport, and importing it can breach medicines law in many countries.
Storage and handling
Lyophilized powder is the stable form. Keep it cold and dark; avoid repeated temperature swings and direct light.
Refrigerate at 2–8 °C, never freeze, and respect the roughly 28-day beyond-use window of the bacteriostatic water. Discard anything cloudy or particulate.
Label every vial with concentration and mix date. Rotate injection sites so the same patch of subcutaneous tissue is not used week after week, and never reuse a needle.
Frequently asked questions
What is retatrutide?
Retatrutide (LY3437943) is an investigational triple agonist that activates the GLP-1, GIP and glucagon receptors. It is being studied by Eli Lilly for obesity and type 2 diabetes. It is not approved by the FDA, EMA, MHRA or TGA, and anything sold online as 'retatrutide' is an unregulated research chemical of unverified identity, purity and sterility.
What retatrutide doses were used in trials?
The Phase 2 obesity trial (NEJM, 2023) tested weekly subcutaneous doses of 1 mg, 4 mg, 8 mg and 12 mg, reached through a slow escalation over several months rather than started outright. Higher arms began at 2 mg and stepped up every 2–4 weeks. These are trial figures under medical supervision with monitoring — not a protocol to copy.
How much weight did people lose in the retatrutide trial?
At 48 weeks, mean weight reduction was about 8.7% on 1 mg, 17.1% on 4 mg, 22.8% on 8 mg and 24.2% on 12 mg, versus roughly 2.1% on placebo. Importantly the curves had not flattened at 48 weeks, so the ceiling is unknown. Individual results in the trial varied widely around those means.
How is retatrutide titrated?
Trial titration was deliberately gradual because gastrointestinal side effects track dose escalation speed more than the final dose. Participants stayed at each step for at least 2–4 weeks before moving up, and steps were held or reduced when nausea, vomiting or dehydration appeared. Skipping steps is the most common cause of people abandoning a GLP-1-class compound.
How do you reconstitute retatrutide?
Lyophilized vials are reconstituted with bacteriostatic water. Concentration in mg/mL equals the vial strength in mg divided by the milliliters of BAC water added — so a 10 mg vial with 2 mL gives 5 mg/mL, and a 1 mg dose is 0.2 mL, which is 20 units on a U-100 insulin syringe. Add the diluent slowly against the vial wall, swirl rather than shake, and refrigerate once mixed.
How long does reconstituted retatrutide last?
Peptides reconstituted with bacteriostatic water are generally kept refrigerated at 2–8 °C and used within about 28–30 days, the same beyond-use window that applies to the bacteriostatic water itself. Plain sterile water has no preservative and is a single-use diluent. Discard anything cloudy, discolored or containing particles, and never freeze a reconstituted vial.
How long until you see results?
In trial data appetite suppression usually appears within the first one to two injections, while measurable weight change lags behind it. Most participants were still losing weight at 48 weeks, so this is a multi-month arc rather than a fast cut. Judging the compound after three or four weeks at a starting dose tells you almost nothing.
Retatrutide vs tirzepatide — what's actually different?
Tirzepatide is a dual GLP-1/GIP agonist; retatrutide adds glucagon-receptor agonism, which raises energy expenditure on top of appetite suppression. In separate Phase 2 trials retatrutide's highest arm produced roughly 24% mean weight loss at 48 weeks versus about 21% for tirzepatide at 72 weeks. The trials were never run head to head, so the comparison is indirect. Tirzepatide is approved and prescribable; retatrutide is not.
Retatrutide vs semaglutide?
Semaglutide is a single GLP-1 agonist and the most established of the three, with approved obesity and diabetes indications plus cardiovascular outcome data. Retatrutide's trial weight-loss figures are higher, but it has no long-term safety record, no outcome data and no approval. More receptors also means more mechanisms that can misbehave.
What are the side effects?
Predominantly gastrointestinal: nausea, vomiting, diarrhea and constipation, worst during escalation. Dose-dependent increases in heart rate were seen in trials, as were transient rises in fasting glucose at the highest doses via the glucagon arm. Muscle loss alongside fat loss is a real concern with this magnitude of weight reduction, which is why protein intake and resistance training matter.
Does retatrutide cause muscle loss?
Any rapid, large weight loss costs lean mass — across GLP-1-class trials roughly a quarter to 40% of total weight lost has been fat-free mass, some of which is water and glycogen rather than contractile muscle. The levers that reduce it are unchanged: adequate protein (broadly 1.6 g per kg of target body weight per day), resistance training two to four times a week, and not escalating faster than you can eat.
What happens if you stop?
Appetite returns. In the tirzepatide withdrawal trial (SURMOUNT-4) participants switched to placebo regained about 14% of body weight over the following year while those who continued lost more. No retatrutide withdrawal data exists yet, but there is no reason to expect a different pattern. Weight-regulating compounds work while they are being used.
What if you miss a weekly dose?
Trial protocols for weekly GLP-1-class agents generally allow a missed dose to be taken if the next scheduled dose is more than about 72 hours away, otherwise skip it and resume the normal schedule. Never double up to catch up — stacking doses concentrates exactly the gastrointestinal effects that escalation is designed to avoid. After several missed weeks, tolerance fades and restarting at a lower step is typical.
Who should not use it?
Anyone with a personal or family history of medullary thyroid carcinoma or MEN2, prior pancreatitis, severe gastroparesis, active gallbladder disease, or who is pregnant, breastfeeding or planning pregnancy. GLP-1-class compounds also slow gastric emptying, which changes absorption of oral medications including oral contraceptives.
What monitoring makes sense?
Baseline and periodic fasting glucose and HbA1c, lipids, liver and kidney function, and resting heart rate. Weight and waist alone hide muscle loss — body composition or at least grip strength and lifting numbers give a better picture. Anyone using an unapproved compound should be doing this with a physician who knows about it.
Is retatrutide legal to buy?
It is not approved for human use anywhere. Vendors sell it labeled 'for research use only', which is a legal shield, not a quality guarantee. Third-party testing has repeatedly found research peptides that are underdosed, misidentified or contaminated. It is also prohibited in tested sport.
References & sources
Peer-reviewed trial publications and regulatory guidance cited on this page. Last reviewed 2026-08-02.
- Jastreboff AM, Kaplan LM, Frías JP, et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial. N Engl J Med. 2023;389(6):514–526. https://pubmed.ncbi.nlm.nih.gov/37366315/
- Rosenstock J, Frias J, Jastreboff AM, et al. Retatrutide in people with type 2 diabetes: a randomized, double-blind, placebo- and active-controlled, phase 2 trial. Lancet. 2023;402(10401):529–544. https://pubmed.ncbi.nlm.nih.gov/37385280/
- Coskun T, Urva S, Roell WC, et al. LY3437943, a novel triple glucagon, GIP, and GLP-1 receptor agonist for glycemic control and weight loss: preclinical and Phase 1 results. Cell Metab. 2022;34(9):1234–1247. https://pubmed.ncbi.nlm.nih.gov/35985340/
- Jastreboff AM, Aronne LJ, Ahmad NN, et al. Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1). N Engl J Med. 2022;387(3):205–216. https://pubmed.ncbi.nlm.nih.gov/35658024/
- Aronne LJ, Sattar N, Horn DB, et al. Continued Treatment With Tirzepatide for Maintenance of Weight Reduction (SURMOUNT-4). JAMA. 2024;331(1):38–48. https://pubmed.ncbi.nlm.nih.gov/38078870/
- Wilding JPH, Batterham RL, Calanna S, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). N Engl J Med. 2021;384(11):989–1002. https://pubmed.ncbi.nlm.nih.gov/33567185/
- U.S. Food and Drug Administration. Medications Containing Semaglutide Marketed for Type 2 Diabetes or Weight Loss — compounded and unapproved product warnings. https://www.fda.gov/drugs/postmarket-drug-safety-information-patients-and-providers/medications-containing-semaglutide-marketed-type-2-diabetes-or-weight-loss
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Start free trialEducational reference only, not medical advice. Retatrutide is not an approved medicine — do not start, stop or combine any protocol without a qualified physician.