Compounded Retatrutide: Dosing, Research Data, and Sourcing Guide (2026)

Retatrutide (LY3437943) is the next molecule in the injectable weight loss pipeline after tirzepatide. Where tirzepatide added a second receptor -- GIP alongside GLP-1 -- retatrutide adds a third: the glucagon receptor. That triple mechanism is why Phase 2 trial data published in the New England Journal of Medicine showed mean weight loss of 24.2% at 48 weeks in the highest dose arm, the largest average weight loss number recorded in any injectable obesity trial at the time of publication. For context, tirzepatide's Phase 2 top dose reached approximately 20% at a similar time point.

Retatrutide is not FDA-approved as of mid-2026. Eli Lilly's TRIUMPH Phase 3 program is ongoing. Research-grade retatrutide is available from peptide vendors for laboratory use and is being studied in both academic and independent research settings. This guide covers the mechanism, the Phase 2 data, what research protocols look like, and what quality standards matter when evaluating a source.

What Is Retatrutide?

Retatrutide is a synthetic peptide that acts as a triple agonist at three G-protein-coupled receptors: GLP-1 (glucagon-like peptide-1), GIP (glucose-dependent insulinotropic polypeptide), and the glucagon receptor. Eli Lilly developed it under the internal designation LY3437943.

The GLP-1 and GIP components are the same dual mechanism found in tirzepatide -- GLP-1 slows gastric emptying, reduces appetite, and stimulates insulin secretion; GIP works on fat tissue and the central nervous system to layer on additional satiety and energy-regulation effects. What retatrutide adds is glucagon receptor agonism, which is mechanistically distinct:

• Hepatic fat oxidation: Glucagon receptor activation increases fat burning in the liver, reducing hepatic fat content and contributing to overall caloric expenditure beyond what GLP-1 and GIP achieve alone.
• Energy expenditure: Glucagon is a counterregulatory hormone that increases metabolic rate. Low-level glucagon receptor activation on top of GLP-1/GIP signaling appears to push total energy expenditure higher without the full glycemic disruption of pharmacologic glucagon doses.
• NASH / liver effects: The glucagon component makes retatrutide a candidate for non-alcoholic steatohepatitis (NASH) research beyond its primary obesity indication, a secondary interest that has driven research attention to the molecule.

The key open question in the field is whether the glucagon receptor activity produces a proportional benefit on weight loss at therapeutic doses or primarily adds tolerability challenges. The Phase 2 data suggests meaningful additive benefit, but Phase 3 data across a larger population will be more definitive.

Phase 2 Weight Loss Data

The pivotal Phase 2 data for retatrutide was published in the New England Journal of Medicine in June 2023 (Jastreboff et al.). The trial enrolled 338 adults with obesity (BMI 30 or above) over 48 weeks of treatment. Dose arms were 1 mg, 4 mg, 8 mg, and 12 mg weekly subcutaneous injection, each with a titration phase before reaching the target dose.

The dose-response relationship was steep between 1 mg and 8 mg, with the incremental benefit flattening somewhat between 8 mg and 12 mg. This pattern is consistent with receptor saturation dynamics and suggests that 8 mg may be a clinically efficient dose for many research subjects, with the 12 mg dose providing meaningful but diminishing additional benefit.

Phase 3 TRIUMPH trial data, expected to fully read out in 2026 and 2027, will provide the larger-population efficacy and safety picture. Phase 3 trials typically enroll several thousand participants with longer follow-up periods than Phase 2.

How Research Protocols Approach Dosing

No FDA-approved dosing protocol exists for retatrutide. The Phase 2 titration schedule -- which is the primary public reference -- used a slow multi-step escalation before reaching the target dose. The general structure for research purposes follows the Phase 2 design:

Phase 2 Escalation Reference (12 mg Target Dose Arm)

Slower escalation, with 6 to 8 week holds at each step rather than 4 weeks, is a common research adjustment to reduce GI side effects. Because retatrutide includes glucagon receptor activity in addition to GLP-1, the nausea and vomiting profile at higher doses is more pronounced than with semaglutide and comparable to tirzepatide. Most researchers hold escalation if GI symptoms are poorly tolerated rather than pushing through on a fixed schedule.

What Research-Grade Retatrutide Is Sold As

Research peptide vendors sell retatrutide in lyophilized (freeze-dried) powder form, requiring reconstitution with bacteriostatic water before use. Common vial sizes are 10 mg, 15 mg, and 30 mg. Pre-mixed solutions are less common for retatrutide than for shorter peptides, in part because of its synthesis complexity and the precision required for higher-dose protocols.

A 30 mg lyophilized vial reconstituted in bacteriostatic water to a concentration of 5 mg/mL yields a 6 mL solution. Working through the escalation from 2 mg to 12 mg weekly over approximately 12 to 16 weeks requires roughly 80 to 100 mg total -- multiple vials for a full protocol cycle. Most researchers source in advance given the multi-month commitment involved.

Storage for lyophilized retatrutide follows the same pattern as other long-chain acylated peptides:

• Before reconstitution: Store at -20C, protected from light and moisture.
• After reconstitution: Refrigerate at 2 to 8C. Use within 28 days. Do not freeze reconstituted solution.
• In transit: Short-term exposure to ambient temperature is generally tolerated by the lyophilized form. Check vendor guidance for specific stability data.

For step-by-step reconstitution instructions, see the peptide reconstitution guide.

How to Evaluate a Retatrutide Source

Retatrutide is a more challenging synthesis target than most research peptides. It is a longer-chain peptide with a fatty acid modification that drives its extended half-life, similar in design philosophy to semaglutide and tirzepatide. Synthesis errors, incomplete reactions, or impurities in the fatty acid conjugation step can produce a peptide that passes basic visual inspection but has reduced receptor binding activity or unexpected impurity profiles.

Third-Party COA

A certificate of analysis from an independent testing laboratory is non-negotiable. For retatrutide specifically, the COA should include:

• HPLC purity: 98% or higher from an independent lab, not in-house testing. Retatrutide's complexity means that 95% "purity" hides more potentially active impurities than it would for a simpler peptide.
• Mass spectrometry identity confirmation: Verifies the correct molecular weight and peptide identity. Given that retatrutide is similar in structure to tirzepatide and other GLP-1 class peptides, identity verification is particularly important to confirm you have the correct molecule.
• Batch-specific documentation: The COA should reference the specific batch number that matches your shipment. A single historical COA applied to all inventory is a red flag.

Salt Form Disclosure

Research peptide vendors typically sell retatrutide as the acetate salt. Confirm the salt form disclosed on the COA before cross-referencing any dosing protocol, as free base, acetate, and TFA salt forms have slightly different weights per unit dose.

Synthesis Transparency

Vendors who disclose their synthesis method -- solid-phase peptide synthesis (SPPS) followed by solution-phase fatty acid conjugation, the standard approach for acylated GLP-1 class peptides -- demonstrate product knowledge. Vendors who cannot answer questions about synthesis method or who rely entirely on third-party resellers with no insight into the manufacturing chain warrant more scrutiny.

Side Effects: What Phase 2 Data Shows

The side effect profile from the Phase 2 trial is the primary public safety reference. GI effects dominate, consistent with the GLP-1 and GIP receptor activity:

• Nausea: The most common adverse event, reported in 42 to 55% of participants at the higher dose arms during escalation periods. Typically self-limiting within 4 to 8 weeks at any given dose step.
• Diarrhea: Reported in 20 to 35% across dose arms, more common at 8 mg and 12 mg.
• Vomiting: Reported in 15 to 25% at higher doses. A meaningful step up from tirzepatide's Phase 2 data, which showed approximately 10 to 15% across comparable dose arms.
• Constipation: Reported in approximately 10% across dose arms, lower than the nausea frequency.
• Decreased appetite: The primary intended effect; frequently described by participants as the compound working rather than an adverse experience.

The discontinuation rate due to adverse events in the Phase 2 trial was approximately 11% in the 12 mg arm -- higher than comparable Phase 2 data for tirzepatide, which was approximately 6% at the highest dose. The higher rate is attributed to the GI burden at 8 mg and 12 mg, particularly during escalation. Slower titration with extended holds at each step reduces but does not eliminate this effect.

The glucagon receptor component of retatrutide adds a theoretical hyperglycemia risk if dosed in a way that uncouples the glucagon signal from the GLP-1/GIP insulin-stimulating effects. In the Phase 2 trial design, this did not produce clinically meaningful glucose disruption, but it is a mechanistic consideration that distinguishes retatrutide from the dual agonists.

For a comparison of side effect profiles across the GLP-1 drug class, see the tirzepatide side effects guide and the semaglutide side effects guide.

Retatrutide vs the Current Weight Loss Peptide Landscape

Retatrutide occupies a distinct position in the 2026 research peptide market: it has Phase 2 data showing the highest weight loss efficacy of any GLP-1 class molecule, it is in active Phase 3 development, and it is not FDA-approved. That combination means high research interest and no branded product to draw access away from the research market.

The direct comparison between retatrutide and tirzepatide that most researchers are interested in does not yet exist as a head-to-head trial. The efficacy gap suggested by the Phase 2 data may narrow or widen in Phase 3. For a deeper look at the current state of the head-to-head question, see the tirzepatide vs retatrutide guide.

Regulatory Context

Retatrutide does not carry the same compounding pharmacy regulatory complexity as semaglutide and tirzepatide, both of which were placed on the FDA drug shortage list and then became subjects of FDA guidance letters narrowing compounding permissions as shortages eased. Because retatrutide is not yet an approved drug, it does not exist in the pharmaceutical compounding framework at all.

Research-grade retatrutide from peptide vendors is sold for in vitro and laboratory research use. It is not subject to the 503A/503B compounding pharmacy regulations that apply to approved drug compounding. It is, however, subject to FDA enforcement priorities regarding research chemicals sold in a manner that implies human use or makes medical claims. Vendors operating in this space have increasingly moved toward clearer research-use disclaimers, COA transparency, and product presentation that reflects laboratory rather than clinical intent.

Researchers should be aware that if retatrutide receives FDA approval through the TRIUMPH program, the regulatory and market environment for research-grade access could shift, as occurred with semaglutide and tirzepatide. Monitoring FDA announcements from the TRIUMPH program and any NDA filings from Eli Lilly is relevant for long-range research planning.