Tirzepatide vs Retatrutide: Approval, Evidence and Lab Results

The main difference is regulatory and biological at once: tirzepatide activates two hormone receptors (GIP and GLP-1) and is FDA-approved, while retatrutide activates three (GIP, GLP-1 and glucagon) and is still investigational. Both are Eli Lilly molecules. Retatrutide has now reported Phase 3 results from five trials, and Lilly said on September 29, 2026 that it plans to submit a Biologics License Application to the FDA in the first quarter of 2027. The head-to-head trial of retatrutide against tirzepatide is still running, and no results have been reported, so nobody can yet say from a direct comparison which produces more weight loss.

At a glance

How they work differently

Tirzepatide is built from the sequence of the gut hormone GIP and modified to also activate GLP-1 receptors, and its label describes it as a GIP and GLP-1 receptor agonist (Zepbound prescribing information, DailyMed). Retatrutide is a single molecule that activates those two receptors plus the glucagon receptor (Eli Lilly, "What to know about retatrutide," September 2026). Lilly's page calls "GLP-3" a scientifically inaccurate nickname and prefers "triple agonist."

Glucagon is best known as a hormone that raises blood sugar, so adding it to a weight-loss drug can sound backwards. Why the trials produced what they did, and how much the glucagon part contributes, is a mechanism question the trials weren't designed to answer. What they do show is a set of outcomes and side effects.

What the evidence shows

Head to head: not yet

Lilly is running a Phase 3 trial that compares retatrutide with tirzepatide in adults who have obesity (TRIUMPH-5, NCT06662383). ClinicalTrials.gov lists it as active and no longer recruiting, with an estimated enrollment of 800, a primary endpoint of percent change in body weight at week 80, and an estimated primary completion date of November 2026 (record last verified July 2026). No results are posted. Until they are, there is no direct evidence on which is stronger.

That matters because comparing numbers across separate trials is unreliable. The trials differ in who was enrolled, how long they lasted, how doses were escalated, and how the analysis handled people who stopped treatment. Sponsors often report an "efficacy estimand" (what happens if everyone stays on treatment) in press releases and a "treatment-regimen estimand" in journals, and the two give different numbers. So we won't put one drug's percentage next to the other's and declare a winner.

Tirzepatide on its own

The approval trial for weight management, SURMOUNT-1, enrolled 2,539 adults with obesity or overweight and no diabetes. At the highest dose studied, average weight change at 72 weeks was -20.9% versus -3.1% on placebo (Jastreboff et al., NEJM 2022). Tirzepatide also beat semaglutide in two head-to-head trials against that drug; our semaglutide vs tirzepatide comparison covers them, and the tirzepatide guide has the rest.

Retatrutide on its own

A Phase 2 trial of 338 adults with obesity reported weight-loss results in 2023. At the highest dose studied, average weight change at 48 weeks was -24.2% versus -2.1% on placebo (Jastreboff et al., NEJM 2023).

Phase 3 has since reported. Peer-reviewed results so far include:

• TRIUMPH-2 (1,152 adults with obesity or overweight and type 2 diabetes): at 80 weeks, average weight change with the highest dose studied was -18.8% versus -5.1% on placebo, on the treatment-regimen estimand (Bellido et al., Lancet 2026).
• TRANSCEND-T2D-1 (537 adults with type 2 diabetes not controlled by diet and exercise): at 40 weeks, A1c fell by an average of 1.94 percentage points at the highest dose studied versus 0.81 on placebo, with average weight change of -15.3% versus -2.6% (Bajaj et al., Lancet 2026).

Lilly also reported topline results from TRIUMPH-1, in 2,339 adults with obesity or overweight and no diabetes, in a June 2026 press release: average weight loss of 28.3% at the highest dose after 80 weeks on the sponsor's efficacy estimand, plus improvements in knee osteoarthritis pain and sleep apnea severity in the two sub-studies (Eli Lilly press release, June 6, 2026). Lilly's FAQ says these findings were also published in the New England Journal of Medicine; we cite the press release because we could not confirm that paper in the journal index. See the retatrutide guide for more.

So the honest verdict on evidence: tirzepatide's is stronger in the sense that matters for safety and reliability, because it rests on approved use and multiple large trials, including two against semaglutide. Retatrutide's Phase 3 results are large and consistent, but they are sponsor-run, the drug isn't approved, and the longest data are 80 weeks in the main trials and 104 weeks in a subgroup extension of TRIUMPH-1.

Side effects and risks compared

Tirzepatide's label carries a boxed warning: in rats it caused thyroid C-cell tumors, and it is unknown whether it does in humans. It is contraindicated in people with a personal or family history of medullary thyroid carcinoma or MEN 2, and the label lists acute gallbladder disease and pancreatitis among its warnings (Zepbound label).

Retatrutide has no label. In its trials, the most common side effects were gastrointestinal: nausea, diarrhea, constipation and vomiting. Two other signals stand out. In TRIUMPH-1, dysesthesia (abnormal skin sensations) was reported by 12.5% of people on the highest dose versus 0.9% on placebo, generally mild to moderate and mostly resolving; and discontinuation because of side effects was 11.3% at the highest dose versus 4.9% on placebo, per Lilly's press release. The TRIUMPH-2 paper also reported hypotension and dysesthesia more often on retatrutide than placebo, and the Phase 2 paper reported dose-dependent increases in heart rate that peaked at 24 weeks. Deaths in TRIUMPH-2 were judged unrelated to the study drug by the investigators.

The Zepbound label lists dysesthesia and a small average heart-rate increase too, with dysesthesia under 1% in its pooled trials. Retatrutide's trials reported dysesthesia far more often, but the trials differ in design and population, so read it as a signal to watch rather than a ranking. Those signals aren't proof of harm or safety in either direction. They mean retatrutide's safety picture is still being built, while tirzepatide's is documented across a much larger group of people.

Then there's the vial risk. Lilly's page states that retatrutide "is legally available only through Lilly's clinical trials" and warns that illicit products may contain unknown ingredients or the wrong amount of active ingredient. Lilly has also opened a narrow expanded-access (compassionate-use) pathway, announced August 3, 2026, for adults who cannot join a trial and meet strict criteria (ClinicalTrials.gov NCT07629401), so its own trials are no longer the only authorized route. Tirzepatide has an approved version to compare against; retatrutide doesn't, so a research-labeled vial is the only form on the grey market.

What the lab tests show

These figures come from independent lab reports, mostly commissioned and submitted by buyers, as of September 30, 2026, with unidentified vendors excluded. VendorVerify doesn't test anything itself. A vial counts as failed if it was empty, under 95% pure, more than 20% under label or more than 50% over.

Tirzepatide: 788 vials from 48 vendors (March 2023 to September 2026). Median purity 99.76%, with 709 of 784 at 99% or higher and 0 under 95%. Median measured amount 5.5% over label; of 761 vials with a label and measured amount, 309 were within 5%. 9 were more than 20% under, 51 more than 20% over (3 more than 50% over), and 4 were empty. Failed: 16 (2.0%).

Retatrutide: 341 vials from 37 vendors (October 2024 to September 2026). Median purity 99.70%, with 322 at 99% or higher and 7 under 95%. Median measured amount 8.2% over label; of 295 vials with both amounts, 94 were within 5%. 6 were more than 20% under, 30 more than 20% over, and 1 more than 50% over. Failed: 14 (4.1%).

As of September 30, 2026, tirzepatide has more than twice as many vials, and a few vendors can move vial-level percentages, so treat the gap between the two failure rates as a snapshot. Both compounds' purity has mostly come back high; the differences are in fill amount, where both skew over label.

Here's who has the strongest independent lab record for tirzepatide right now:

And for retatrutide:

Buying smarter

Check each compound's record on its own results page, /peptides/tirzepatide and /peptides/retatrutide, rather than a vendor's overall reputation. Our test-results articles go deeper on tirzepatide and retatrutide, and can you buy retatrutide in the USA covers the legal picture. Learn to read a peptide COA and check for fake-COA patterns. If you want your own answer, get your vial tested. If a report comes back bad, stop using that batch, tell anyone who bought from the same run, take it to the vendor, and submit it to VendorVerify.

Database figures without a date update automatically from VendorVerify's lab results; dated figures are snapshots. Trial status is from ClinicalTrials.gov and Eli Lilly as of September 30, 2026. This isn't medical advice, and research-labeled versions of both compounds are sold for research use only.