Retatrutide: the triple agonist, explained

What retatrutide is

Retatrutide (development code LY3437943) is a synthetic peptide from Eli Lilly that activates three gut/metabolic hormone receptors at once: the receptors for glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-1 (GLP-1), and glucagon. The first two are the same pair that tirzepatide hits; the third — glucagon-receptor activation — is associated with increased energy expenditure and more aggressive hepatic fat oxidation in trials, which is the mechanistic story behind retatrutide being described as a "triple G" agonist. It is engineered for a long duration of action, which is why it's dosed once weekly.

It is the first triple hormone-receptor agonist to reach Phase 3 with published human data. It is not an approved drug — there is no brand name, no prescription form, and no pharmacy-grade product. People search for it precisely because it is unavailable through legitimate channels and is instead sold gray-market as a "research chemical." That distinction matters for everything below.

What the trials showed

Retatrutide's Phase 2 obesity results were published in the New England Journal of Medicine in 2023, showing dose-dependent weight reduction that drew attention as among the strongest seen for an incretin compound. Lilly then advanced it into the Phase 3 TRIUMPH program — a set of pivotal trials enrolling thousands of participants across obesity and type 2 diabetes indications.

In TRIUMPH-4 (reported December 2025), participants on the 12 mg weekly dose had a mean body-weight reduction of about 28.7% at 68 weeks, alongside significant relief of knee osteoarthritis pain. In TRIUMPH-1 (reported May 2026), 12 mg participants lost an average of roughly 28% of body weight over 80 weeks, with a higher-BMI sub-group in an extension reaching about 30% at 104 weeks. These figures are published trial outcomes for specific protocol doses under medical supervision — they are cited here as facts, not as a target or a recommendation.

Reported tolerability mirrored the GLP-1 class — predominantly gastrointestinal (nausea, vomiting, diarrhea), generally dose-related — with a modest heart-rate increase noted, consistent with the glucagon component. Read the primary sources linked at the foot of this page rather than relying on this summary.

The math

Because retatrutide ships (in the research market) as a lyophilized powder, any use involves reconstitution arithmetic — the same arithmetic the peptide reconstitution calculator does for any vial. Worked example, presented purely as arithmetic:

That's it — concentration is just milligrams divided by millilitres, and units are just hundredths of a millilitre on a U-100 insulin syringe. Change the water volume and every number moves: the same 10 mg vial with 1 mL gives 10 mg/mL (so 0.5 mg = 5 units), while 4 mL gives 2.5 mg/mL (finer markings, larger draws). Rather than do this by hand, the reconstitution calculator converts vial size + water + target into exact syringe units (and works in reverse — units + target → water needed). None of this implies a dose you should take; it's the conversion math for whatever a clinician or trial protocol specifies.

Half-life & what a weekly schedule means

Lilly's Phase 1/Phase 2 pharmacokinetic data put retatrutide's elimination half-life at roughly 6 days — long, because the peptide carries a fatty-diacid tail that binds albumin and slows clearance. A ~6-day half-life is what makes once-weekly dosing viable: by the time the next dose is due, a little over half the prior dose remains, so levels stack toward a plateau rather than spiking and crashing.

The practical consequence is steady state. With weekly dosing and a ~6-day half-life, blood concentration climbs for about 4–5 weeks before it stabilizes — which is also why incretin trials titrate slowly and why effects (and side effects) at a new dose aren't fully expressed on week one. You can see this accumulation curve for any half-life and interval in the half-life visualizer, which plots single-dose and steady-state curves so "weekly, ~6-day half-life" becomes an actual shape instead of an abstraction.

Storage & handling

These are general handling facts for lyophilized peptides, stated as stability practice — not an endorsement of any product:

• Lyophilized (powder, unreconstituted): kept frozen for long-term storage; freeze-dried peptide is the most stable form and is typically stored cold and away from light.
• Reconstituted (in bacteriostatic water): refrigerated at 2–8°C (36–46°F), not frozen — freeze/thaw cycles can damage peptide bonds. Published stability practice assigns GLP-class peptides in bacteriostatic water a beyond-use window of roughly four weeks.
• Reconstitution technique: standard aseptic practice is to swab the stopper, add water slowly down the vial wall (not onto the powder), and swirl gently rather than shake until dissolved.
• BUD honesty: a "beyond-use date" is a stability convention, not a sterility guarantee. With unregulated material, neither label accuracy nor sterility can be assumed — which is the core risk of the gray market.

Tracking retatrutide in OptiPin

If you are tracking a weekly GLP-class compound, OptiPin treats it like any other entry — without endorsing it:

• Dose log + weekly reminders — record each injection on a once-weekly cadence and get reminded on the due day, with site rotation.
• Weight & body metrics — log weight and measurements and see them plotted directly over your dose timeline, so a trend lines up against actual dosing weeks.
• OptiInsight analysis — OptiPin's AI reads the full record (doses, weight, symptoms, labs) and surfaces what moved with what, instead of you eyeballing separate charts.
• Vial & supply tracking — log the reconstituted vial; OptiPin estimates remaining volume, doses left, and an empty date, and warns before you run out.
• Built-in reconstitution math — the same calculator covered above is in the app, so the syringe-unit conversion is one tap, not arithmetic on paper.