GLP3-R

GLP3-R is retatrutide (LY3437943), Eli Lilly's triple hormone receptor agonist with simultaneous activity at GLP-1R, GIPR and GCGR. Comparative context: semaglutide (GLP-1R only) produces roughly 15% average weight loss, tirzepatide (GLP-1R/GIPR) roughly 20%, retatrutide 24.2% at 48 weeks in Phase 2 and 28.7% at 68 weeks at the top dose in Phase 3.

Half-life is approximately 6 days, supporting once-weekly subcutaneous dosing; steady state arrives after approximately four to five half-lives, roughly 4 to 5 weeks of consistent dosing. The acyl fatty diacid moiety at lysine-17 is what extends the half-life (Li et al., 2024).

Regulatory status as of early 2026: not FDA approved, in Phase 3 via the TRIUMPH programme (over 5,800 participants across four studies). TRIUMPH-4 reported December 2025. FDA submission planned for late 2026 or early 2027, potential approval late 2027. Lilly has pursued a biologics license application rather than a standard drug application, which would grant up to 12 years of market exclusivity versus 5, and is building a $27 billion manufacturing facility in anticipation of demand.

Administration: subcutaneous, once weekly, fixed day, preferably a consistent time; fasting not required. Reconstitution in bacteriostatic water, with concentration determined by total peptide content divided by water volume. Four vial sizes are covered. Syringes: 1 mL, 29 or 30 gauge, 5/16 inch tip for dosing; 3 mL, 23–27 gauge, 1 inch tip for reconstitution. Anyone unfamiliar with unit measurement or reconstitution should read the prep and injection guide before proceeding.

The GLP2-T entry notes that GLP2-T reached near-equivalent total weight loss to GLP3-R but over 72 weeks versus 48.

Triple receptor coverage, with different roles per arm.

GLP-1R. Postprandial incretin signalling: glucose-dependent insulin secretion, delayed gastric emptying, reduced hepatic glucose output, and hypothalamic satiety signalling — the tonic appetite drive rather than meal termination.

GIPR. Retatrutide is approximately 9 times more potent at GIPR than native GIP. Effects: augmented postprandial insulin secretion, blunted glycaemic excursions, and nutrient partitioning towards uptake and use by adipose and muscle. Tirzepatide body composition data showed only about 25% of weight lost as muscle versus 39 to 45% for semaglutide, attributed in part to GIPR-mediated partitioning.

GCGR. The differentiating arm. Glucagon receptor activation drives hepatic fatty acid oxidation, raises resting energy expenditure, and promotes lipolysis. This adds to the expenditure side of energy balance rather than only the intake side.

Potency is deliberately unbalanced: approximately 9 times native potency at GIPR but only about 40% of semaglutide's GLP-1R potency, which is why subjective appetite suppression is the weakest of semaglutide, tirzepatide and retatrutide despite superior weight loss. Users measuring efficacy by hunger underestimate the compound, since hepatic fat oxidation is not a sensation.

Activation follows sigmoidal dose-response, not a threshold. By EC50, GIPR is the most sensitive of the three, GLP-1R intermediate, GCGR the least sensitive, so increasing dose progressively recruits glucagon activity. In the Phase 2 liver fat sub-study, beta-hydroxybutyrate rose two to three times above baseline at doses of 4 mg and above, coinciding with the largest DEXA-measured fat loss step. "Glucagon kicks in at 4 mg" is a practical simplification of where the signal becomes measurable, not a pharmacological threshold.

Cryo-EM work (Li et al., 2024) showed the N-terminal nine residues are conserved across all three receptor interactions while residues 10 to 21 vary to enable receptor-specific binding.

Carbohydrate intake is not required: hepatic fatty acid oxidation, lipolysis and raised resting energy expenditure all run on stored fat, and glucagon drives gluconeogenesis from amino acids and glycerol when glycogen is depleted. Low-carb diets elevate glucagon endogenously, so the combination is synergistic. The one caveat is ketone production: Phase 3 added a ketoacidosis warning, relevant to strict keto plus high-dose retatrutide plus a stressor such as illness, to diabetics, and to those on SGLT2 inhibitors. At moderate low carb (50 to 100 grams) this is a non-issue.

Efficacy profile, by trial.

Weight. Phase 2 (Jastreboff et al., 2023): 338 adults with obesity, 48 weeks — 8.7% at 1 mg, 17.1% at 4 mg, 22.8% at 8 mg, 24.2% at 12 mg, 2.1% placebo. At 12 mg, about 63% lost at least 20% and roughly 25% lost 30% or more; weight was still declining at week 48. Phase 3 TRIUMPH-4 (December 2025), 68 weeks: 26.4% at 9 mg, 28.7% at 12 mg (approximately 71 pounds on average), 2.1% placebo, with approximately 39% of the 12 mg arm losing 30% or more. Response rates at 8 mg and 12 mg in Phase 2: 100% achieved at least 5% loss; at 12 mg, 93% at least 10% and 83% at least 15%.

Hepatic steatosis. Phase 2a MASLD sub-study (Sanyal et al., 2024), 98 participants with at least 10% liver fat: relative reduction at 24 weeks of 42.9% at 1 mg, 57.0% at 4 mg, 81.4% at 8 mg and 82.4% at 12 mg versus a 0.3% increase on placebo. Normal liver fat (below 5%) in 86% at 12 mg versus 0% placebo, with reductions correlating to body weight, abdominal fat, insulin sensitivity and lipid metabolism.

Glycaemia and cardiometabolic markers. Phase 2 T2D trial (Rosenstock et al., 2023), 281 adults over 36 weeks across 42 U.S. centres: HbA1c reduction of 1.39% at 4 mg, 1.99% at 8 mg, 2.02% at 12 mg at 24 weeks; up to 82% reached HbA1c below 6.5%; 16.94% weight loss at 36 weeks at the top dose versus 3.0% placebo and 2.02% dulaglutide; no severe hypoglycaemia and no deaths. Systolic blood pressure fell approximately 9.88 mmHg in meta-analysis (Abouelmagd et al., 2025), which also reported 14.33% mean body weight reduction, BMI −5.38, waist circumference −10.51 cm, fasting plasma glucose −23.51 mg/dL and HbA1c −0.91% across three RCTs totalling 878 patients. TRIUMPH-4 confirmed non-HDL cholesterol and systolic blood pressure improvements.

Body composition. Coskun et al., 2025: 12 mg produced a 23.2% reduction from baseline in total fat mass in T2D adults versus 2.6% for dulaglutide, with the ratio of lean mass loss to total weight loss similar to other obesity treatments — a greater proportion of lean mass is not lost despite greater total loss.

Energy expenditure. GCGR activation raises resting energy expenditure, so the compound acts on expenditure as well as intake.

Osteoarthritis. TRIUMPH-4: WOMAC pain scores reduced by up to 75.8%, with more than 1 in 8 treated patients completely free from knee pain at trial end.

Weeks 1 to 4 (2 mg). Appetite suppression is mild relative to semaglutide or tirzepatide, consistent with roughly 40% of semaglutide's GLP-1R potency. Mild GI effects — nausea, constipation — may appear in week one and typically resolve within days. Reduction in food noise is subtle at this stage.

Weeks 5 to 12 (4 mg then 8 mg). Metabolic effects become pronounced and weight loss accelerates; 1.5 to 2 pounds per week is typical at moderate doses. Glucagon-driven hepatic fat oxidation and raised resting energy expenditure become measurable at 4 mg and above (beta-hydroxybutyrate two to three times baseline) without being subjectively perceptible. Blood pressure normalisation is often noted in this window.

Week 13 onward (8 to 12 mg). Steady, substantial loss at 1.5 to 2 pounds per week or more, with continued progression in glycaemic, blood pressure and lipid markers. Weight was still declining at week 48 in Phase 2 and through week 68 in Phase 3, so extended use continues to yield results.

Dose-dependent adverse profile. GI events dominate: Phase 2 nausea 25 to 45% by dose, with diarrhoea, vomiting, constipation and decreased appetite all common; TRIUMPH-4 at the highest dose reported nausea approximately 43%, diarrhoea approximately 33%, vomiting approximately 21%. Dysesthesia/allodynia occurred in 7 to 21% overall (8.8% at 9 mg and 20.9% at 12 mg versus 0.7% placebo), presenting as tingling, pressure and heat hypersensitivity and the sensation of clothing on skin, typically worse at night and attributed to GLP-1 receptors in the brain stem, spinal cord and pain-processing nerve clusters. Discontinuation for adverse events was 12.2% at 9 mg and 18.2% at 12 mg. Heart rate rose, peaking at week 24 before declining. Hair thinning tracks rapid weight loss and undereating rather than the drug.

Reported in practice. 10 to 20 pounds in the first month at 2 mg, partly water and glycogen; 1.5 to 2 pounds per week sustained at therapeutic doses; 25 to 30% total reduction within a year for many. Food indifference is common enough that users set reminders to eat, and protein targets become the limiting factor. Roughly 10 bpm higher resting heart rate is reported by those tracking it. Feeling cold is widespread, sometimes alongside a core temperature around 99 degrees Fahrenheit. Improved sleep quality and normalised blood pressure are also reported. Dysesthesia is reported by a minority and described as the most uncomfortable effect; dose reduction, split dosing, zinc (15 to 30 mg zinc picolinate daily) or antihistamine (10 mg loratadine daily) are used against it, none tested in trials for this indication.

Adjuncts that determine composition outcome. One gram of protein per pound of goal body weight daily, resistance training a minimum of 2 to 3 days per week, daily walking, and sleep. A PMC case series of patients on GLP-1 medications who prioritised resistance training 3 to 5 days per week with adequate protein documented one patient losing 33% of body weight with only 6.9% muscle loss and another gaining 2.5% muscle while losing 26.8% body weight.

Studied doses. 1 mg, 4 mg, 8 mg (slow and fast escalation), 9 mg and 12 mg weekly subcutaneous across Phase 2 and Phase 3. Phase 2 demonstrated that initiation at 2 mg with slow escalation produced significantly fewer GI adverse events at similar 48-week weight loss versus higher initiation.

Dose-response with diminishing returns. 4 mg weekly approximately 17% at 48 weeks; 8 mg approximately 23%; 12 mg approximately 24% in Phase 2 and 28.7% at 68 weeks in Phase 3. The 4 mg to 8 mg step added about 5 percentage points; 8 mg to 12 mg added about 2 in Phase 2.

Titration (Phase 2 design, best tolerability). Weeks 1 to 4 at 2 mg, weeks 5 to 8 at 4 mg, weeks 9 to 12 at 8 mg, week 13 onward at 12 mg maximum, once weekly on a fixed day. Hold any step a further 4 weeks if tolerability is poor; there is no benefit to pushing through severe nausea.

Goal-matched targets. 4 to 8 mg may suffice for a 15 to 20% target; 8 to 12 mg for 20% or more. Prior semaglutide or tirzepatide users may exceed their previous maximum-dose results at moderate retatrutide doses.

Exposure-based initiation. GLP-naive: 2 mg weekly. Prior exposure to any GLP with at least one escalation after starting (thresholds of 0.50 mg semaglutide or 5 mg tirzepatide), regardless of time elapsed: 4 mg weekly.

Split dosing. Total weekly dose drives steady-state concentration, not injection frequency, so 2 mg weekly and 1 mg twice weekly give the same average steady-state level. Splitting compresses the peak-to-trough ratio, stabilises serum levels and can mitigate GI effects and peak-related dysesthesia. This is distinct from microdosing, defined pharmacologically as less than 1/100th of a pharmacologically active dose; genuine microdosing would never reach a meaningful steady state.

Duration and cycling. Not mandatory. Phase 2 ran 48 weeks, Phase 3 runs 68 weeks, weight still declining at the end of both; most run to goal weight. A 4-month on, 4-month off pattern is an option for anyone wanting to help ensure sustained efficacy.

Reconstitution. Concentration equals total peptide content divided by bacteriostatic water volume. Vial configurations: 10 mg / 1 mL = 10 mg/mL; 30 mg / 2.5 mL = 12 mg/mL; 40 mg / 2.5 mL = 16 mg/mL; 60 mg / 2.5 mL = 24 mg/mL. At 10 mg/mL the maximum single 1 mL draw is 10 mg, so the 12 mg maintenance step requires a larger vial or a more concentrated reconstitution. Syringes: 1 mL, 29–30 G, 5/16 inch for dosing; 3 mL, 23–27 G, 1 inch for reconstitution.

Do not add another GLP-1 agonist. Semaglutide or tirzepatide on top duplicates the GLP-1R arm and raises side-effect burden without proportional benefit; the pathway-additive option for insufficient appetite suppression is cagrilintide.

Phase 1 through Phase 3 data are published, making this the most robustly evidenced investigational obesity agent covered here.

Phase 2 obesity (Jastreboff et al., 2023, NEJM): 338 adults with obesity, 48 weeks. Mean weight loss 8.7% at 1 mg, 17.1% at 4 mg, 22.8% at 8 mg, 24.2% at 12 mg, versus 2.1% placebo. At 12 mg roughly 63% lost at least 20% and about 25% lost 30% or more; at 8 mg and 12 mg, 100% of participants achieved at least 5% loss, with 93% at ≥10% and 83% at ≥15% at 12 mg. Weight was still declining at week 48. Initiation at 2 mg versus 4 mg produced significantly fewer side effects with comparable 48-week outcomes.

Phase 2 type 2 diabetes (Rosenstock et al., 2023, The Lancet): 281 adults, 36 weeks, 42 US centres. HbA1c reductions at 24 weeks of 1.39% (4 mg), 1.99% (8 mg), 2.02% (12 mg); up to 82% reached HbA1c below 6.5%. Weight loss 16.94% at the top dose versus 3.0% placebo and 2.02% dulaglutide. No severe hypoglycaemia, no deaths.

Phase 2a MASLD sub-study (Sanyal et al., 2024, Nature Medicine): 98 participants with at least 10% hepatic fat, 48 weeks. Relative liver fat change at 24 weeks: −42.9% (1 mg), −57.0% (4 mg), −81.4% (8 mg), −82.4% (12 mg) versus +0.3% placebo; 86% reached hepatic fat below 5% at 12 mg versus 0% placebo. Reductions correlated with body weight, abdominal fat, insulin sensitivity and lipid metabolism. The same sub-study found beta-hydroxybutyrate rising two to three times above baseline at doses of 4 mg and above, aligning with the largest DEXA-measured fat loss step and marking where glucagon-receptor recruitment becomes measurable.

Body composition sub-study (Coskun et al., 2025, Lancet Diabetes and Endocrinology): 23.2% reduction from baseline in total fat mass at 12 mg versus 2.6% for dulaglutide, with lean-to-total loss proportion similar to other obesity treatments.

Phase 3 TRIUMPH-4 (December 2025): obesity or overweight with knee osteoarthritis, 68 weeks. 26.4% at 9 mg, 28.7% at 12 mg (approximately 71 pounds), 2.1% placebo. WOMAC pain reduced up to 75.8%, more than 1 in 8 completely free of knee pain; non-HDL cholesterol and systolic blood pressure improved. Dysaesthesia 8.8% at 9 mg and 20.9% at 12 mg versus 0.7% placebo; AE discontinuation 12.2% and 18.2%.

Meta-analysis (Abouelmagd et al., 2025, Baylor University Medical Center Proceedings): three RCTs, 878 patients. Weight −14.33% mean difference, BMI −5.38, waist −10.51 cm, fasting plasma glucose −23.51 mg/dL, HbA1c −0.91%, systolic BP −9.88 mmHg, with no significant difference in overall adverse events versus placebo.

Structural work (Li et al., 2024, Cell Discovery): cryo-EM structures show simultaneous binding at GLP-1R, GIPR and GCGR; the conserved N-terminal nine residues drive all three interactions while residues 10 to 21 confer receptor selectivity, and the acyl fatty diacid at lysine-17 extends the half-life to approximately 6 days.

Regulatory status as of early 2026: not FDA approved, in Phase 3 via TRIUMPH with over 5,800 participants across four studies, submission planned for late 2026 or early 2027 and potential approval late 2027 under a biologics license application carrying up to 12 years of exclusivity versus 5.