Retatrutide reconstitution calculator

Enter your vial amount and target dose. DrawDose returns the BAC water volume, syringe units, and a vial label you can copy.

What retatrutide is and where it stands legally

Retatrutide (development code LY3437943) is an investigational once-weekly subcutaneous peptide developed by Eli Lilly. Per Eli Lilly's company communications, it is a first-in-class triple hormone receptor agonist that simultaneously activates GLP-1, GIP, and glucagon receptors — distinguishing it from semaglutide (GLP-1 only) and tirzepatide (GLP-1 + GIP). The molecule is a 39-amino-acid synthetic peptide built on a GIP backbone with a C20 fatty diacid conjugation that enables albumin binding and once-weekly dosing.

Retatrutide is not FDA-approved. As of April 2026, Eli Lilly has not yet submitted a New Drug Application. The TRIUMPH Phase 3 trial program is ongoing, with seven additional readouts expected throughout 2026. Per the December 2025 Eli Lilly press release on TRIUMPH-4 and the March 2026 TRANSCEND-T2D-1 readout, the Phase 3 efficacy and safety data have so far supported the Phase 2 results. FDA submission is expected in late 2026 or 2027, with potential approval projected for 2027–2028.

The legal access landscape today: clinical trial enrollment is the only fully sanctioned path. Compounded retatrutide is not authorized under the 503A bulks list. Research-grade retatrutide sold through peptide vendors is not FDA-cleared for human use and per FDA Final Guidance is presumed intended for human use when shipped to residential addresses regardless of label disclaimers.

This page covers the dosing math and protocols documented in published Phase 2 and Phase 3 trial data plus community protocol surveys. It is reportage of what the literature shows, not a recommendation to obtain or use retatrutide outside FDA-sanctioned channels.

Quick answer for the most common vial sizes

The 4 mL : 10 mg ratio documented in compounded-pharmacy formulation guides for retatrutide produces a 2.5 mg/mL concentration, where every 40 units delivers 1 mg.

The math holds at any concentration. DrawDose accepts any vial size and any BAC water volume and returns the correct syringe draw for the dose entered.

For doses above 8 mg per injection (relevant to the 9 mg and 12 mg maintenance doses studied in TRIUMPH-4), a more concentrated reconstitution is necessary to keep the syringe draw under 100 units. The 10 mg/mL ratio (achieved with 20 mg in 2 mL or 10 mg in 1 mL) is the practical default for the highest dosing tiers.

How to reconstitute lyophilized retatrutide

The procedure follows the standard sterile-injection prep workflow documented in compounding pharmacy formulation guides and peptide stability literature.

Bring the vial and BAC water to room temperature for 15 to 20 minutes before mixing. Cold liquid hitting cold powder slows dissolution and increases the chance of clumping. Wipe both vial tops with an alcohol swab and let them dry. Draw the BAC water into a 1 mL syringe.

Insert the needle into the retatrutide vial at an angle so the water runs down the inside wall, not directly onto the powder pellet. Direct impingement on lyophilized peptide generates foam and can affect peptide structure. Once the water is in, swirl the vial gently for 20 to 30 seconds. Do not shake. Retatrutide dissolves clearly when properly reconstituted; cloudiness or particulate that does not clear within a minute indicates a sourcing concern.

Why concentration determines syringe units

The relationship between vial size, BAC water, and syringe draw is fixed by concentration math. A 10 mg vial with 4 mL of water gives 2.5 mg/mL — every 40 units delivers 1 mg. The same 10 mg vial with 2 mL of water gives 5 mg/mL, where 40 units delivers 2 mg. To hit the same 2 mg target dose, the draw is either 80 units (at 2.5 mg/mL) or 40 units (at 5 mg/mL).

Standard reconstitution practice keeps the draw between 10 and 100 units on a 1 mL insulin syringe. Retatrutide's wide titration range (0.5 mg to 12 mg per injection across published protocols) means the choice of reconstitution ratio matters more than for narrower-dosed peptides. Lower starting doses reconstitute well at 2.5 mg/mL; higher maintenance doses need 5 or 10 mg/mL to stay within syringe capacity. DrawDose computes this automatically; the auto-selected BAC water volume on the result panel is tuned to keep the draw in a measurable band.

Phase 2 titration schedule

Per the Jastreboff et al. Phase 2 trial published in NEJM in 2023, retatrutide was administered as a once-weekly subcutaneous injection with progressive titration. The published titration schedule for the highest dose arm (12 mg target) was:

Weeks 1 to 4: 2 mg once weekly. The Phase 2 starting dose; lower starting doses (0.5 mg, 1 mg) were also evaluated in separate arms.

Weeks 5 to 8: 4 mg once weekly.

Weeks 9 to 12: 8 mg once weekly.

Weeks 13 onward: 12 mg once weekly (maintenance for the highest-dose arm).

Other arms in the trial used different escalation patterns and reached lower maintenance doses (4 mg, 8 mg). The Phase 3 TRIUMPH-4 trial evaluated 9 mg and 12 mg maintenance doses; the TRIUMPH program also includes a 4 mg maintenance dose arm with results expected in 2026.

The half-life of retatrutide is approximately 6 days per the Phase 1 pharmacokinetic data referenced in the Phase 2 publication, supporting the once-weekly dosing schedule. Steady-state plasma concentrations are reached after approximately 4 weeks of consistent weekly dosing.

Community protocols: slow titration and microdosing

Survey data aggregated from r/Retatrutide and broader GLP-1 community forums describes titration patterns that deviate from the published trial schedules.

Slow titration extends each step from the published 4 weeks to 6 or 8 weeks. This pattern is documented among users with persistent gastrointestinal effects. Reaching the 12 mg dose can take 24 to 32 weeks under slow titration instead of the 12 weeks the trial protocol specified.

Microdosing starts below 0.5 mg, often at 0.25 mg or lower, with extended step-ups before joining the standard titration line. There is no published clinical trial data on retatrutide doses below 0.5 mg. Some users in survey data report extended periods at sub-trial doses (1 mg, 2 mg) without escalating, accepting a smaller weight loss outcome in exchange for tolerability.

The reconstitution math is unaffected by which titration pattern is followed. DrawDose accepts any target dose and returns the unit draw at the chosen concentration.

Phase 3 results documented to date

Per the December 2025 Eli Lilly press release, the TRIUMPH-4 Phase 3 trial in adults with obesity and knee osteoarthritis (without diabetes) reported 28.7% average body weight reduction at 68 weeks at the highest dose tested. Per the March 2026 TRANSCEND-T2D-1 press release, the Phase 3 trial in type 2 diabetes met its primary endpoints for HbA1c reduction and weight loss. Detailed results from both trials are scheduled for presentation at medical meetings and peer-reviewed publication.

The Phase 2 data published by Jastreboff et al. in 2023 showed up to 24.2% body weight reduction at 48 weeks in the highest-dose arm in adults with obesity. The Phase 3 results have so far been consistent with or stronger than the Phase 2 efficacy.

Comparator context per published trials and meta-analyses: semaglutide (Wegovy) reaches approximately 15% mean body weight reduction at maintenance dose; tirzepatide (Zepbound) reaches approximately 20-22%. Retatrutide's reported 24-29% at maintenance dose is the highest weight loss reported in any GLP-1 class clinical trial to date.

Adverse reactions per Phase 2 and Phase 3 publications

Per the Jastreboff et al. 2023 Phase 2 publication, the most common adverse reactions were nausea, diarrhea, vomiting, and constipation. Frequency was dose-dependent. Nausea was reported in 35% to 64% of participants depending on dose, with most cases mild to moderate and concentrated in the early titration period.

Heart rate elevation was a notable signal in retatrutide trials beyond what is typical for semaglutide or tirzepatide. The Phase 2 publication reported small but measurable increases in resting heart rate at higher doses, attributed to the glucagon receptor agonism component. Per the December 2025 TRIUMPH-4 announcement, the Phase 3 safety profile was consistent with the Phase 2 data and with the broader GLP-1 class. Detailed cardiovascular safety data is anticipated from the dedicated cardiovascular outcomes trial in the TRIUMPH program.

Boxed warning context: the GLP-1 class carries a boxed warning for thyroid C-cell tumors based on rodent carcinogenicity studies for semaglutide and tirzepatide. Whether retatrutide will carry the same warning depends on its rodent carcinogenicity data and the FDA's class-effect determination at approval. As of April 2026, no boxed warning has been finalized because the drug is not yet approved.

Other documented adverse reactions across the GLP-1 class that may apply to retatrutide include acute pancreatitis, gallbladder disease, acute kidney injury (associated with severe gastrointestinal effects), and diabetic retinopathy complications in patients with type 2 diabetes. Long-term safety data on retatrutide specifically does not exist beyond the Phase 3 trial durations of 48 to 68 weeks.

Common reconstitution errors

Compounded peptide forums and pharmacy QA literature document a recurring set of errors specific to retatrutide.

Confusing milligram and microgram dosing. Retatrutide is dosed in milligrams (0.5 mg, 1 mg, 2 mg, 4 mg, 8 mg, 12 mg). Some users default to thinking in micrograms because other peptides are dosed in mcg; this leads to 1000-fold dosing errors. Always verify the unit before calculating any draw.

Reusing the same reconstitution ratio across the entire titration. The 2.5 mg/mL ratio that works well at 1 mg or 2 mg doses produces inconveniently large syringe draws (up to 480 units) at 12 mg target. The math has to be recalculated as titration progresses; a more concentrated reconstitution is appropriate for higher dosing tiers.

Reusing reconstituted vials beyond 4 to 6 weeks. Per peptide stability literature, reconstituted retatrutide remains stable for approximately 4 to 6 weeks at 2 to 8°C with bacteriostatic water containing benzyl alcohol preservative. After that, peptide degradation accelerates and concentration drift becomes meaningful.

Trusting the vial label over the COA. Compounded and research-grade vials run 5 to 15% off label in industry-published variance data. The COA reports the actual measured peptide content per the lot's HPLC and mass spec assays.

Storing reconstituted vials in the refrigerator door. Door storage cycles through temperature variation each time the door opens. The back of the main compartment is the documented recommendation in peptide stability literature.

Storage and shelf life

Per peptide stability literature, reconstituted retatrutide stays stable for 4 to 6 weeks at 2 to 8°C (36 to 46°F) when reconstituted with bacteriostatic water. The benzyl alcohol preservative is what enables this; sterile water without preservative produces a 24-hour shelf life from first puncture.

Lyophilized retatrutide stores for 24 months or longer at -20°C and 12 months or longer at 2 to 8°C, per the same stability literature. Heat exposure during shipping is the most documented threat to potency; vendors that ship without cold-pack insulation in summer months are flagged by community survey data as producing reduced-potency product.

What to verify on a Certificate of Analysis

The COA reports the actual peptide content of a specific lot. Net peptide weight is the measured milligrams of peptide in the vial, separate from any excipients (mannitol, sodium chloride, phosphate buffer) included for stability. Purity by HPLC reflects the percentage of UV-absorbing material that is the target peptide; 95% is the research-grade minimum the peptide industry has converged on, and 98 to 99% is the standard most reputable vendors publish for retatrutide.

Mass spec confirmation matches the expected molecular weight (4,731.33 Da for retatrutide) and verifies the peptide's identity. The C20 fatty diacid conjugation is detectable by mass difference and is the primary check against substitution with unmodified GIP backbone or with shorter-acting GLP-1 analogs mislabeled as retatrutide.

Documented combinations

Combination protocols are reported in compounded-pharmacy literature and community surveys; none have published FDA-aligned trial data as combinations.

Retatrutide combined with semaglutide or tirzepatide is not documented as a stable practice. All three compounds are GLP-1 receptor agonists with overlapping mechanisms; co-administration produces the side effect load of multiple compounds without proportional efficacy gain. Sequential use (transitioning from semaglutide or tirzepatide to retatrutide) is documented in clinical-practice guidelines for users seeking the deeper weight loss documented in Phase 2 and Phase 3 trials.

BPC-157 is occasionally added in community protocols for managing retatrutide-related gastrointestinal effects, with the rationale that BPC-157's gastric mucosa effects may offset retatrutide's delayed gastric emptying side effects. The combination is community-reported with no peer-reviewed support.

FAQ

Is retatrutide FDA-approved?

No. As of April 2026, retatrutide is an investigational drug in Phase 3 clinical trials. Eli Lilly has not submitted it for FDA approval. Approval is projected for 2027 or 2028 based on standard regulatory timelines.

How is retatrutide different from tirzepatide?

Tirzepatide (Mounjaro, Zepbound) is a dual agonist of GLP-1 and GIP receptors. Retatrutide adds glucagon receptor agonism, making it a triple agonist. Per Phase 2 and Phase 3 trial data, the additional glucagon component appears to drive higher weight loss (24-29% vs tirzepatide's 20-22% at maintenance dose) but also produces small heart rate increases not seen with tirzepatide.

What is "GLP-3"?

"GLP-3" is an informal nickname for retatrutide that emerged in media coverage. There is no actual GLP-3 hormone. The nickname references retatrutide's three-receptor mechanism (GLP-1 + GIP + glucagon). Eli Lilly uses the technical term "triple agonist" rather than GLP-3.

How can I legally access retatrutide today?

Per FDA guidance and ClinicalTrials.gov, the only fully sanctioned access path is enrollment in an active TRIUMPH Phase 3 trial. Multiple sites globally are recruiting. Trials provide retatrutide free of cost to enrolled participants but require meeting enrollment criteria (BMI thresholds, comorbidity profile, location).

Is retatrutide banned by WADA?

As of April 2026, retatrutide is not specifically listed on the WADA prohibited list because it is not yet a marketed drug. WADA's S2 category (Peptide Hormones, Growth Factors, Related Substances and Mimetics) covers GLP-1 receptor agonists; once retatrutide gains FDA approval and enters clinical use, athletes subject to anti-doping testing should expect inclusion under that category. Verify the current WADA list before assuming status.

How does retatrutide compare to semaglutide for weight loss?

Per published clinical trial data, semaglutide (Wegovy) produces approximately 15% mean body weight reduction at maintenance dose over 68 weeks. Retatrutide produced 24-29% mean reduction in Phase 2 and Phase 3 trials over similar durations. Direct head-to-head trials between retatrutide and semaglutide have not been published; the comparison is based on separate trials with similar but not identical design.

What's the difference between Phase 2 and Phase 3 retatrutide dosing?

The published Phase 2 trial (Jastreboff et al. 2023) tested 1, 4, 8, and 12 mg target maintenance doses. The Phase 3 TRIUMPH-4 trial tested 9 and 12 mg. The TRIUMPH program also includes a 4 mg maintenance dose arm with results expected later in 2026. The dose ranges overlap; the Phase 3 program is establishing safety and efficacy at the higher end of the dose-response curve documented in Phase 2.