# Script Retatrutide — A clear-eyed brief on an investigational triple-agonist > Independent editorial brief on retatrutide (LY3437943), the investigational GIP/GLP-1/glucagon tri-agonist in Phase 3 trials. What the data show. Why it cannot be prescribed or compounded today. Retatrutide (LY3437943) is a once-weekly peptide that activates three nutrient-hormone receptors at once. The Phase 2 data are striking. The Phase 3 program is underway. There is no pathway, today, by which a clinician can write you a script for it — and the reasons are worth understanding. ## The one-paragraph version Retatrutide is a once-weekly injectable compound being studied by Eli Lilly under the code name LY3437943. It works by activating three hormone receptors at once — GLP-1 (appetite), GIP (insulin), and glucagon (calorie-burning). No approved drug does all three simultaneously. In the largest Phase 2 study, participants with obesity lost an average of 24% of their body weight over 48 weeks at the highest dose, compared to 2% on placebo. That is a striking number, and the Phase 3 program is now running to confirm it at scale. The important catch: retatrutide is not FDA-approved, cannot be prescribed, and cannot be lawfully compounded. This brief explains what the published trials show, what the regulatory framework actually permits, and what the evidence does and does not yet establish. [See what people report on the effects page.](/effects) ## What retatrutide is Retatrutide is a single 39-residue synthetic peptide that simultaneously activates three receptors involved in metabolism and appetite: GIPR (glucose-dependent insulinotropic polypeptide receptor), GLP-1R (glucagon-like peptide-1 receptor), and GCGR (the glucagon receptor) [6]. The first two of those receptors should be familiar by now — they are the targets that semaglutide and tirzepatide work through. The third, glucagon, is what makes retatrutide structurally new. The molecule carries a C20 fatty-diacid tail that binds reversibly to circulating albumin, which is the trick that stretches its plasma half-life to roughly six days and lets it dose once weekly by subcutaneous injection [5]. An aminoisobutyric-acid substitution at the N-terminus protects it from DPP-IV, the enzyme that ordinarily clears native GLP-1 within minutes. Development is led by a single sponsor under the code name LY3437943. As of 2026 it has not received marketing approval from any regulatory authority anywhere in the world. ## What the data show The Phase 2 obesity trial, published in *NEJM* in 2023, is the readout most people have heard about. Adults with obesity and without type 2 diabetes lost a least-squares mean of 24.2% of baseline body weight at 48 weeks on the 12 mg dose, versus 2.1% on placebo [1]. The dose-response was linear all the way through the highest arm — at 24 weeks the 1, 4, 8, and 12 mg cohorts had lost 7.2%, 12.9%, 17.3%, and 17.5% respectively, with no plateau in sight [2]. A parallel Phase 2 trial in type 2 diabetes published in *The Lancet* the same year reported HbA1c reductions of roughly 2.0 percentage points and weight loss of about 16.9% at 24 weeks at the 12 mg dose [3]. A Phase 2a substudy in adults with metabolic dysfunction–associated steatotic liver disease (MASLD) reported an 86.0% relative reduction in liver fat by MRI-PDFF at 48 weeks on the 12 mg dose, with roughly 90% of participants achieving resolution of hepatic steatosis [4]. In December 2025 the first Phase 3 readout — TRIUMPH-4, in adults with obesity plus knee osteoarthritis — reported -28.7% mean weight loss at 68 weeks on the 12 mg arm and a 75.8% reduction in WOMAC pain scores [8]. The picture across these studies is consistent. The numbers are larger than anything previously reported for GLP-1 mono-agonists or GLP-1/GIP dual agonists in matched populations. That does not, by itself, mean the compound is approved, safe across long-term cardiovascular endpoints, or available. ## Why a doctor cannot write a script for it A prescription is a clinician's authorization to dispense a drug the FDA has approved for marketing. Retatrutide has no FDA approval for any indication. It has not been submitted for one. The earliest realistic timeline for a New Drug Application is late 2026, contingent on the general-population Phase 3 obesity trial (TRIUMPH-1) and the cardiovascular outcomes trial (TRIUMPH-3) reading out [11, 12]. When a drug has no approval, it can lawfully be administered to humans only inside an authorized clinical trial under an Investigational New Drug exemption. Outside that setting, a physician who 'prescribes' retatrutide is not engaged in off-label use — there is no label to be off. They are directing the use of an unapproved new drug, which is a different legal category and not one that a state medical license alone permits. This is a meaningfully different situation from the case of tirzepatide or semaglutide, both of which carry FDA-approved labels for type 2 diabetes and chronic weight management. Off-label use of an approved drug is a routine, lawful, and well-understood practice. Use of an unapproved investigational drug outside a trial is a separate framework with separate rules. See /research for the full data summary, /references for citations, and /faq for the questions most readers arrive with. ## Why compounding pharmacies cannot supply it either The most common follow-up question is whether a compounding pharmacy can prepare retatrutide the way some prepared semaglutide and tirzepatide during the 2022–2024 shortages. The answer, settled by the FDA in 2025, is no. The statutory pathways are FDCA Section 503A (traditional pharmacy compounding) and 503B (registered outsourcing facilities). To compound from a bulk drug substance under either pathway, the substance must meet at least one of three criteria — it must be the subject of an applicable USP or NF monograph, it must be a component of an FDA-approved drug, or it must appear on the relevant 503A or 503B bulks list. Retatrutide meets none of those. It has also never been on the FDA drug shortage list, which closes the additional 503A shortage pathway used during the GLP-1 shortages [16]. On 9 September 2025 the FDA issued more than 40 warning letters, several of which explicitly named retatrutide as an investigational substance that 'cannot be used in compounding under federal law' [17]. Any compounded retatrutide product distributed in the United States is therefore, by definition, an unapproved new drug. This site is editorial commentary on that landscape, not legal advice — but the regulatory facts are not in dispute. --- An independent editorial digest of peer-reviewed research — not a clinic, not a prescriber, not a vendor.