No trial has yet compared retatrutide against tirzepatide head to head and published the result. That single fact should shape how you read every comparison you find, including this one. Eli Lilly is running the direct trial right now — NCT06662383, 800 adults with obesity, retatrutide against tirzepatide as an active comparator, primary endpoint percent weight change at week 80 — and its estimated primary completion date is November 2026. Until that reads out, nobody knows which drug wins.
What we do know is that one of them is a medicine you can be prescribed today and the other is not. Tirzepatide has been FDA-approved since 2022 and sells as Mounjaro for type 2 diabetes and Zepbound for weight management and obstructive sleep apnea. Retatrutide has no approved product record in Drugs@FDA. Lilly runs a pre-approval expanded access program for it, restricted to adults with a BMI of 35 or higher plus two or more serious obesity-related complications who cannot get into a trial. That is the profile of a drug close to filing, not a drug you can pick up at a pharmacy.
Tirzepatide is a dual agonist. One molecule activates the GIP receptor and the GLP-1 receptor. Retatrutide adds a third: GIP, GLP-1, and glucagon. Both are once-weekly subcutaneous injections.
The glucagon arm is the whole story. On its own, glucagon raises blood sugar, which is the opposite of what you want in a diabetes drug. Paired with GIP and GLP-1 activity in the same molecule, the thinking is that glucagon agonism adds energy expenditure and liver fat reduction while the other two receptors keep glucose in check. TRANSCEND-T2D-1, the phase 3 monotherapy trial in type 2 diabetes published in the Lancet in June 2026, supports the "kept in check" half of that: HbA1c fell 1.94 percent from baseline at the 12 mg dose over 40 weeks, against 0.81 percent on placebo, and no severe hypoglycemia was reported.
If you want the fuller background on the molecule itself, we cover it separately in our retatrutide preview. For where tirzepatide sits against the other approved option, see tirzepatide vs semaglutide.
Here is what each drug has actually reported, with the trial attached:
Retatrutide, phase 2, 338 adults with obesity, 48 weeks. Least-squares mean weight change was -8.7 percent at 1 mg, -17.1 percent at 4 mg, -22.8 percent at 8 mg and -24.2 percent at 12 mg, against -2.1 percent on placebo. At 12 mg, 83 percent of participants lost 15 percent or more of body weight.
Tirzepatide, SURMOUNT-1, phase 3, 2,539 adults with obesity, 72 weeks. Mean weight change was -15.0 percent at 5 mg, -19.5 percent at 10 mg and -20.9 percent at 15 mg, against -3.1 percent on placebo. At 15 mg, 57 percent lost 20 percent or more.
Read casually, that is 24.2 versus 20.9 and retatrutide wins. Read carefully, it is a 338-person phase 2 dose-finding study against a 2,539-person phase 3 registrational trial, at different durations, in different populations, with different statistical estimands. Phase 2 numbers routinely shrink in phase 3. Everyone in the field expects some regression here; the only question is how much.
There is one attempt to put both on a common scale. The BMJ published a network meta-analysis in July 2026 covering 262 trials and 99,791 participants. Standardized to one year against lifestyle modification alone, tirzepatide showed a mean difference of -14.9 percent, on moderate-to-high certainty evidence. Retatrutide was grouped with other emerging agents at 13.1 to 14.6 percent, on very low to low certainty evidence. That is not retatrutide losing. It is retatrutide having thin evidence at the one-year mark and being modelled conservatively because of it, and it is a useful corrective to the 24 percent figure that circulates without its context.
Lilly's TRIUMPH program has now finished several trials. TRIUMPH-1 in obesity without diabetes (2,335 participants) reached primary completion in April 2026. TRIUMPH-3 in severe obesity with established cardiovascular disease (1,946 participants) completed the same month. TRIUMPH-2 in obesity with type 2 diabetes (1,152 participants) completed in June 2026.
As of this writing, results for those three trials are not posted on ClinicalTrials.gov and we could not locate the primary publications in PubMed. Review articles have begun referencing phase 3 TRIUMPH data in general terms, so topline figures may be in the public domain through conference presentations or company disclosures. We are not going to quote a number we cannot trace to a primary source. If you see a specific TRIUMPH-1 percentage cited anywhere, check whether it links to a published trial report or to a press release, because those are not the same thing.
The trial that actually answers "retatrutide vs tirzepatide" is NCT06662383. It started in November 2024, runs about 89 weeks, and measures percent weight change from baseline to week 80 in both arms. Estimated primary completion is November 2026, and publication typically follows completion by several months to a year.
Both drugs produce the same category of problem. In the retatrutide phase 2 trial, the most common adverse events were gastrointestinal, dose-related, mostly mild to moderate, and partially reduced by starting at 2 mg instead of 4 mg. In SURMOUNT-1, the most common adverse events with tirzepatide were also gastrointestinal, mostly mild to moderate, and concentrated during dose escalation. In TRANSCEND-T2D-1, discontinuation for adverse events ran 2 to 5 percent on retatrutide against 0 percent on placebo; in SURMOUNT-1, it ran 4.3 to 7.1 percent on tirzepatide against 2.6 percent on placebo. Different trials, so treat that as two separate observations rather than a comparison.
Two things are specific enough to flag. Retatrutide's phase 2 trial reported dose-dependent heart rate increases that peaked around week 24 and declined afterward. And the BMJ meta-analysis found tirzepatide reduced fat mass the most of any drug analyzed, at 25.7 percent, while also reducing lean mass the most, at 8.3 percent. We have no comparable lean mass figure for retatrutide, which is a real gap given that glucagon agonism affects energy expenditure. If you are already dealing with the nausea end of this class, our guides to tirzepatide nausea and protecting muscle on a GLP-1 cover the day-to-day management.
Tirzepatide's label sets out its escalation: Zepbound starts at 2.5 mg once weekly for four weeks, increases in 2.5 mg steps at intervals of at least four weeks, with maintenance at 5, 10 or 15 mg and a maximum of 15 mg. Mounjaro is indicated for glycemic control in type 2 diabetes in adults and in children aged 10 and older. Which dose you sit at, and how fast you get there, is a decision for your prescriber, not something to read off a chart. Our tirzepatide dosing schedule walks through what the label actually says.
Retatrutide has no label, because it has no approval. Trial doses have been 1, 4, 8 and 12 mg weekly in phase 2 obesity, and 4, 9 and 12 mg in TRANSCEND-T2D-1. There is no approved maintenance dose, no titration schedule, and no pharmacy supply chain. Anything sold as retatrutide outside a clinical trial or the expanded access program is not a prescription medicine, has no FDA-reviewed label behind it, and carries none of the manufacturing guarantees that apply to Mounjaro or Zepbound.
The practical answer is that there is nothing to switch to. Retatrutide is not available by prescription, its phase 3 efficacy has not been published in full, and the trial that compares the two directly has not finished. A drug that beats yours in a trial you cannot read is not yet a reason to change anything.
If tirzepatide has stopped doing what it used to, that is a different problem with its own answers, and it is worth working through before waiting on a drug that may be a year or more from a pharmacy shelf. Our 2026 GLP-1 medication list covers what is actually approved and prescribable today.
Is retatrutide stronger than tirzepatide? The phase 2 headline number is higher, at -24.2 percent over 48 weeks versus -20.9 percent over 72 weeks in SURMOUNT-1. But those are different trials at different phases, and phase 2 results commonly moderate in phase 3. The BMJ network meta-analysis, which standardizes to one year, put retatrutide in the same range as tirzepatide with much weaker certainty. The honest answer is that it is unknown until NCT06662383 reports.
When will retatrutide be approved? No approval date is public. There is no retatrutide record in Drugs@FDA, and Lilly runs a pre-approval expanded access program, which indicates the drug is still investigational. Several phase 3 trials completed in 2026 and others, including the cardiovascular and kidney outcomes trial, run to 2029. Anyone giving you a firm approval date is guessing.
Can I get retatrutide legally right now? Only through a clinical trial or Lilly's single-patient expanded access program, which requires a BMI of 35 or higher, two or more serious obesity-related complications, failure of the highest tolerated dose of approved therapy, and inability to enroll in a trial. Your physician applies on your behalf. There is no legitimate retail route.
Does the extra glucagon receptor cause different side effects? The reported side effect profile in phase 2 looked like the rest of the class: gastrointestinal, dose-related, worst during escalation. The one signal that stands apart is the dose-dependent heart rate increase peaking around week 24. Whether glucagon agonism produces distinct long-term effects is exactly what the larger phase 3 trials and the cardiovascular outcomes trial are built to answer.
Should I wait for retatrutide instead of starting tirzepatide? That is a conversation for your prescriber, but the timeline is worth weighing. Tirzepatide is approved, studied in tens of thousands of people, and available now. Retatrutide is not approved, has no published phase 3 obesity results we can verify, and has not been compared against tirzepatide in a completed trial. Waiting means waiting on an unknown quantity for an unknown length of time.