Fat Loss & Metabolism

Retatrutide

LY3437943 · "the Hard R" · triple agonist

research mainstream subQ injection

What it is

Retatrutide is the most advanced GLP-1-class weight-loss compound currently in development — an Eli Lilly investigational drug in late-stage clinical trials. It's a triple agonist, meaning it activates three different metabolic receptors at once: GLP-1, GIP, and glucagon. That third receptor (glucagon) is what makes it different from Ozempic (semaglutide, GLP-1 only) and Mounjaro (tirzepatide, GLP-1 + GIP).

How it works

Each of the three receptors does something useful:

GLP-1 receptor — suppresses appetite, slows stomach emptying, improves insulin sensitivity. (What Ozempic does.)
GIP receptor — further improves insulin response, may enhance fat oxidation, reduces some GLP-1 nausea.
Glucagon receptor — increases resting energy expenditure (you burn more calories doing nothing) and pulls fat out of the liver.

The glucagon arm is what gives Retatrutide its outlier weight-loss numbers — you're not just eating less, you're also burning more.

Benefits

Largest weight loss of any pharmaceutical agent ever tested in trials — ~24% body weight reduction at 48 weeks (12 mg dose)
Reverses fatty liver disease (NAFLD/MAFLD) — ~80% relative reduction in liver fat
Reduces visceral fat aggressively
Improves cardiovascular markers (BP, lipids, A1c)
Potential indication for sleep apnea, knee osteoarthritis, and metabolic disease in trials

Timeline

Week 1–2: Significant appetite suppression. Often nausea during initial titration.
Week 4–8: 5–8% body weight loss typically. Energy expenditure starts increasing (the glucagon effect).
Week 12: 10–15% body weight loss at moderate doses.
Week 24–48: Peak loss — 17–24% depending on dose. Plateau usually hits around month 10.

Dosing & titration

Starting dose2 mg subQ once weekly

TitrationIncrease by 2–4 mg every 4 weeks if tolerating well

Standard ranges4 mg, 8 mg, 12 mg weekly (trial doses)

Max trial dose12 mg/week — rarely needed clinically

When to titrate upOnly if weight-loss has plateaued AND side effects (nausea, constipation) are tolerable. Many users do well at 4–8 mg long-term.

Day to injectSame day each week. Most people pick a day where mild nausea won't disrupt their schedule (often Sunday).

Side effects & risks

Nausea (most common, especially during titration)
Vomiting, diarrhea, or constipation
Fatigue and decreased exercise tolerance
Hair shedding (months 3–6, usually temporary)
Significant muscle loss if not paired with protein + resistance training
Heart rate elevation (5–10 bpm typical)
Rare: gallstones from rapid weight loss, pancreatitis

Critical warnings: Eat enough protein (1g/lb of bodyweight) and lift weights. GLP-1 weight loss without these is up to 40% lean tissue. Contraindicated in personal/family history of medullary thyroid carcinoma or MEN-2. Not yet FDA-approved — only available via compounding pharmacies in the US, status changes frequently.

Typical price

$300–$700/mo From a 503A compounding pharmacy. Higher doses cost more. Will likely launch retail at $1,000+/mo when FDA-approved.

Studies

Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial — The landmark NEJM paper showing 24.2% weight loss at 48 weeks (12 mg dose). NEJM New England Journal of Medicine, 2023
Retatrutide Phase 2 trial in metabolic dysfunction-associated steatotic liver disease (MASLD) — Showed up to 80% relative reduction in liver fat. Nature Medicine Nature Medicine, 2024
Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial (PubMed) — Detailed efficacy and safety data. PubMed PubMed, 2023
Eli Lilly press release: Phase 2 results — Industry summary including safety extension data. Lilly Investor Relations Eli Lilly, 2023

Educational reference only. Not medical advice. Retatrutide is investigational — not FDA-approved as of 2026. Phase 3 trials ongoing. Use only via licensed prescriber and compounding pharmacy.