Retatrutide

What it actually is

Retatrutide is a single molecule that activates three receptors at once: GLP-1, GIP and glucagon. Eli Lilly developed it under the designation LY3437943; the nickname "Triple G" comes from the three targets. It is administered once weekly by subcutaneous injection and has a half-life of roughly six days.

The lineage is easy to follow. Semaglutide hits one receptor. Tirzepatide hits two. Retatrutide hits three, and the third one changes the arithmetic.

Why the third receptor matters

GLP-1 and GIP both work primarily by reducing how much you eat: appetite suppression, slowed gastric emptying, better insulin response. That is a demand-side intervention.

Glucagon works on the other side of the ledger. Activating the glucagon receptor increases resting energy expenditure, drives hepatic fat oxidation, and mobilises stored fat. The body burns more at rest.

Single and dual agonists reduce intake. The triple reduces intake and raises expenditure. That is the mechanistic reason the numbers are larger, and it is also why liver fat reductions are so pronounced. Phase 2 showed reductions of up to 82%, with over 80% of high-dose participants reaching liver fat normalisation.

The obvious objection to a glucagon agonist is that glucagon raises blood sugar. In retatrutide, the simultaneous GLP-1 and GIP activation counterbalances it, and the clinical data shows glycemic control improving rather than deteriorating.

What the trials actually showed

Phase 2, published in the New England Journal of Medicine, followed 338 adults with obesity for 48 weeks. By dose: placebo −2.1%, 1 mg −8.7%, 4 mg −17.1%, 8 mg −22.8%, 12 mg −24.2%. The 12 mg group also saw waist circumference drop about 20 cm, and 72% of prediabetic participants reached normoglycemia. The weight loss curves had not plateaued at 48 weeks.

One figure worth flagging: a 2026 write-up of the same Phase 2 trial reports the 12 mg result as 22.8% rather than 24.2%. That is an estimand difference, not a contradiction: a different question asked of the same data, roughly "what happened to people who stayed on treatment" versus "what happened to everyone randomised". We publish both rather than silently picking one.

Phase 3 obesity data comes from TRIUMPH-1 (NCT05929066), which completed on 30 April 2026 and was presented at the ADA Scientific Sessions on 16 June 2026: −28.3% mean body weight at 80 weeks on 12 mg, −25.9% on 9 mg, −2.2% on placebo. A pre-specified extension then followed a smaller group to week 104: 532 people who started with a BMI of 35 or more and had finished the 80 weeks, each moved to the highest dose they tolerated (9 or 12 mg). Those who continued on 12 mg reached −30.3%, and those switched over from placebo −19.2% (Lilly topline, 21 May 2026). Treatment-emergent adverse events ran 87–89.2% against 80.7% on placebo. No results have been posted to the trial registry, so those numbers are conference-tier, not peer-reviewed.

TRIUMPH-4 is a different trial. NCT05931367 is the knee-osteoarthritis study, completed November 2025. Attributing the obesity readout to it is one of the most common errors in circulating retatrutide content, including in our own earlier material.

Two more things on the watch list. A urinary-tract-infection safety signal is under discussion in the European Journal of Internal Medicine (23 July 2026). And TRIUMPH-Outcomes, the cardiovascular and kidney trial that would tell you whether any of this translates into fewer heart attacks, does not read out until February 2029.

For context: semaglutide's STEP 1 produced 14.9% at 68 weeks; tirzepatide's SURMOUNT-1 produced 22.5% at 72 weeks.

This is the strongest evidence base in our library. It is also still investigational, approved nowhere as of August 2026, with no legitimate route to obtain it outside a trial or the expanded-access programme.

Running the escalation

Every trial protocol escalated gradually, and the Playbook is blunt about why: starting higher nearly doubles GI side effect rates.

Weeks 1–4 at 2 mg weekly. Weeks 5–8 at 4 mg. Weeks 9–12 at 6 mg. Weeks 13–16 at 9 mg. Week 17 onward at 12 mg. Same day each week, any time of day, with or without food, rotating between abdomen, thigh and upper arm. That is the Phase 3 schedule; the Phase 2 trial went 2 → 4 → 8 → 12, and 8 mg is the rung that was left behind.

Reconstitution is unusually simple because the numbers are round. A 10 mg vial with 1 mL of bacteriostatic water gives 10 mg/mL, so the dose in millilitres is the dose in milligrams divided by ten: 2 mg is 0.2 mL or 20 units, 4 mg is 40 units, 6 mg is 60 units, 9 mg is 90 units.

Twelve milligrams is 1.2 mL, which does not fit in a 1 mL U-100 barrel. The Playbook's answer is to draw 60 units, inject, then draw and inject 60 more from the same vial. A 3 mL syringe with an appropriate needle gauge is the alternative.

The side effect that is specific to this compound

Dysesthesia means abnormal skin sensations: tingling, burning, heightened sensitivity to touch. It affected 20.9% of participants on 12 mg and 8.8% on 9 mg, against 0.7% on placebo. It is not seen with semaglutide or tirzepatide. It was generally mild and rarely caused discontinuation, but it is a real, dose-related, compound-specific signal.

Discontinuation rates were 12.2% at 9 mg and 18.2% at 12 mg, against 4% for placebo. Notably, some participants stopped because they felt they were losing weight too quickly, not because of intolerable side effects.

The honest cautions

The GLP-1 class carries a boxed warning for medullary thyroid carcinoma based on rodent studies. Personal or family history of MTC, or MEN2 syndrome, is a hard contraindication.

On oral medications, be careful what you carry over from the approved drugs. Retatrutide is investigational and has no US label, so it has no interaction section: nobody has published what it does to the absorption of anything. The class-level claim that slowed gastric emptying changes the absorption of every oral medication does not survive contact with the labels that do exist. Semaglutide's states that the injection did not affect absorption of orally administered medications, and lists nine drugs including warfarin and both oral contraceptive components as unaffected. The thyroid figure belongs to oral semaglutide specifically, and the contraceptive instruction belongs to tirzepatide specifically. Neither is a class rule, and neither has been shown for this compound either way.

Because the half-life is long, pregnancy planning requires discontinuing at least two months ahead.

And the structural one: weight returns on discontinuation, across the whole class. This is a maintenance decision, not a course.