Retatrutide vs tirzepatide: what the trials show, and what they can't

Retatrutide vs tirzepatide is a comparison between two Eli Lilly drugs that sit at very different stages. Tirzepatide is approved by the FDA and sold as Zepbound for weight and Mounjaro for type 2 diabetes [Drugs@FDA, NDA 217806] [Drugs@FDA, NDA 215866]. Retatrutide is not approved by the FDA or any other regulator [Lilly, Oct 28, 2026]. In their own trials, retatrutide's top dose produced more weight loss. But no study has compared them directly yet, so nobody can say which works better.

This guide sets the two side by side: how they act, what each one's trials found, the side effects, the head-to-head trial and how you can get each. Every number carries its source and the trial it came from. If you're new to the drug, start with what is retatrutide.

How are retatrutide and tirzepatide different?

The main difference is one receptor. Tirzepatide switches on two hormone receptors, GIP and GLP-1 [Zepbound label, Aug 2026]. Retatrutide switches on those two plus a third, the glucagon receptor [NEJM, Sep 29, 2026]. Glucagon is the new part. In Lilly's studies in mice, the glucagon effect added a rise in energy burned on top of the appetite effects of the other two [Cell Metabolism, 2022]. For the science in plain words, see how retatrutide works.

In practice they look alike. Both are injected under the skin once a week. Tirzepatide's half-life is about 5 to 6 days in people with overweight or obesity [Zepbound label, Aug 2026]. Retatrutide's was about 6 days in an early study [Lancet, 2022].

Sources: [Lilly, Sep 29, 2026] [Drugs@FDA, NDA 215866] [Drugs@FDA, NDA 217806] [Zepbound label, Aug 2026] [Mounjaro label, Aug 2026] [Lancet, 2022].

Zepbound's label covers weight loss and long-term weight maintenance in adults with obesity, or with overweight plus a weight-related condition. It also covers moderate to severe obstructive sleep apnea in adults with obesity [Zepbound label, Aug 2026]. Mounjaro's label covers blood sugar control in type 2 diabetes for adults and children 10 and older. Since August 2026 it also covers lowering the risk of heart attack, stroke and heart-related death in adults with type 2 diabetes at high risk [Mounjaro label, Aug 2026] [Drugs@FDA, NDA 215866].

Retatrutide vs Zepbound: which caused more weight loss?

Here are the two main obesity trials, each run against placebo in adults without diabetes. Both figures count everyone who was randomized, including people who stopped the drug. That makes them the fairest pair available.

On paper, retatrutide's top dose comes out about four points ahead. Treat that gap with care, because the two trials differed in ways that move the numbers:

• Length. TRIUMPH-1 ran 80 weeks; SURMOUNT-1 ran 72 [NEJM, Sep 29, 2026] [NEJM, 2022].
• Starting weight. TRIUMPH-1 volunteers averaged 112.7 kg with a BMI of 40.0 [Lilly, May 21, 2026]. SURMOUNT-1 volunteers averaged 104.8 kg with a BMI of 38.0 [NEJM, 2022].
• Timing. SURMOUNT-1 was published in 2022 and TRIUMPH-1 in 2026 [NEJM, 2022] [NEJM, Sep 29, 2026]. They enrolled different volunteers, years apart.
• Placebo results. People on placebo lost 3.9% in one trial and 3.1% in the other.

There is no head-to-head result, so no "winner" can be read from this table. The trial built to settle it, TRIUMPH-5, is covered below.

You may also see 28.3% quoted for retatrutide. That is Lilly's press release figure for TRIUMPH-1, and it estimates what would have happened if everyone had stayed on the drug [Lilly, May 21, 2026]. It measures something different from the 20.9% for tirzepatide, so putting the two side by side would flatter retatrutide. The full trial-by-trial picture is in retatrutide weight loss results.

Retatrutide vs Mounjaro in type 2 diabetes

People with type 2 diabetes tend to lose less weight on these drugs, and both programs show it. In TRIUMPH-2, adults with obesity and type 2 diabetes lost 18.8% on retatrutide 12 mg over 80 weeks, compared with 5.1% on placebo [Lancet, Sep 29, 2026]. In the matching tirzepatide trial, SURMOUNT-2, people lost 14.7% on 15 mg over 72 weeks, compared with 3.2% on placebo [Zepbound label, Aug 2026].

Blood sugar results look closer. A1C is the standard measure of average blood sugar over about three months. Two 40-week trials enrolled people managing type 2 diabetes with diet and exercise alone:

The same caution applies: these are separate trials, run years apart, with no direct comparison. Retatrutide's other diabetes results are graded by evidence strength in retatrutide benefits.

One difference is settled. Mounjaro's label now includes a heart-risk use in type 2 diabetes. It rests on a large trial against another diabetes drug, dulaglutide, where tirzepatide was shown to be no worse, though Superiority to dulaglutide was not established [Mounjaro label, Aug 2026]. Retatrutide has no heart-outcome result yet. Its outcomes trial, planned for about 10,000 people, is running, with main results estimated for February 2029 [ClinicalTrials.gov, NCT06383390].

Side effects side by side

Both drugs mostly cause stomach and gut side effects, and both cause more of them at higher doses. The table puts the top dose of each beside its own placebo group. The retatrutide figures come from Lilly's TRIUMPH-1 press release [Lilly, May 21, 2026]. The tirzepatide figures come from the Zepbound label, which pools SURMOUNT-1 with the diabetes trial SURMOUNT-2 [Zepbound label, Aug 2026].

Look at the placebo columns first. Nausea on placebo was 14.8% in TRIUMPH-1 but 8% in the tirzepatide pool. When the dummy injection gives such different numbers, the trials were counting side effects differently. The Zepbound label itself warns that side-effect rates from one drug's trials cannot be directly compared to rates in the clinical trials of another drug [Zepbound label, Aug 2026].

Still, two gaps are large enough to note. Dysesthesia, an odd or painful sensitivity of the skin, was reported in 12.5% of people on retatrutide 12 mg [Lilly, May 21, 2026]. On tirzepatide 15 mg it was 0.4% [Zepbound label, Aug 2026]. More people also stopped retatrutide because of side effects.

Heart rate rises a little on both. Zepbound raised heart rate by 1 to 3 beats per minute on average, with no rise on placebo [Zepbound label, Aug 2026]. On retatrutide, the phase 2 paper reported heart rate rising with the dose, peaking at 24 weeks and then falling [NEJM, 2023]. The abstract we could read gives no beats-per-minute figure, so we don't give one. Zepbound carries a boxed warning about thyroid tumors seen in rats [Zepbound label, Aug 2026]. Retatrutide trials such as TRIUMPH-5 shut out people with a personal or family history of a rare thyroid cancer (MTC) or the MEN-2 syndrome [ClinicalTrials.gov, NCT06662383]. Every side effect reported so far is covered in retatrutide side effects.

Is there a head-to-head trial of retatrutide vs tirzepatide?

Yes, but it hasn't reported. It's called TRIUMPH-5, registered as NCT06662383. Here is what the registry record showed on Nov 12, 2026 [ClinicalTrials.gov, NCT06662383]:

• Design: phase 3, randomized, double-blind (neither volunteers nor investigators know who gets which drug).
• Who: adults with obesity; about 800 people planned.
• Main measure: percent change in body weight at week 80.
• Dates: started Nov 1, 2024. The registry estimates the main data collection ends in November 2026.
• Status: active, not recruiting. No results posted.

Lilly calls it an ongoing Phase 3 trial in adults with obesity with or without type 2 diabetes [Lilly, Oct 28, 2026]. The registry title says only "adults who have obesity." Lilly has not said when results will come out.

The only head-to-head result retatrutide has today is against semaglutide (Ozempic), in type 2 diabetes. It comes from a press release and has not been peer reviewed [Lilly, Oct 28, 2026]. That trial is covered in retatrutide vs semaglutide.

Cost and access: one is sold, one isn't

Tirzepatide is a prescription medicine. On Nov 12, 2026, Lilly's own Zepbound page listed self-pay prices of $299 to $699 for a one-month supply of the KwikPen, depending on the dose. Taxes and fees may be added [Lilly, Nov 12, 2026]. Insurance rules vary by plan.

Retatrutide has no price and can't be bought legally. Lilly says it is not on the market and that no one can legally sell it for human use [Lilly, Oct 2026]. Asked about cost, Lilly says only that it plans to price it to reflect the overall value to the healthcare system once trials and review are done [Lilly, Oct 2026]. The retatrutide cost guide covers what is known about money and coverage.

Two legal routes exist: a clinical trial, or Lilly's expanded access program. That program is for adults with a BMI of 35 or more despite the top dose of an approved weight medicine, plus two or more serious complications. The request must come from the treating doctor [ClinicalTrials.gov, NCT07629401]. Products sold online as "retatrutide" are neither; the retatrutide for sale guide explains what they are.

Even the filing route is unsettled. Lilly plans to submit a Biologics License Application in Q1 2027 [Lilly, Sep 29, 2026]. But the FDA has ruled retatrutide is a drug, not a biologic, and Lilly's court appeal was undecided when we checked [7th Cir. filing, Oct 8, 2026]. Tirzepatide itself was approved as an ordinary drug, under a new drug application [Drugs@FDA, NDA 217806]. The retatrutide FDA approval guide follows the timeline.

Is retatrutide better than tirzepatide?

Nobody knows yet, and anyone who tells you otherwise is guessing. Here is what the evidence supports as of Nov 12, 2026.

Where retatrutide looks stronger: average weight loss at the top dose, in separate trials of similar design [NEJM, Sep 29, 2026] [NEJM, 2022].

Where tirzepatide is ahead: it is approved, with a full FDA label, a known price and years of use outside trials. It also has approved uses retatrutide lacks, including sleep apnea and heart-risk reduction in type 2 diabetes [Zepbound label, Aug 2026] [Mounjaro label, Aug 2026]. And in their own trials, fewer people stopped it because of side effects.

What would settle it: TRIUMPH-5, which tests both in the same trial with the same measure.

Is retatrutide better than tirzepatide for you? That question is for a clinician. Today it can only be answered with a drug you can actually be prescribed.

Can you take retatrutide and tirzepatide together?

No trial has tested them together, so there is no evidence on safety or benefit. The Zepbound label says using it with any glucagon-like peptide-1 (GLP-1) receptor agonist is not recommended [Zepbound label, Aug 2026]. Retatrutide acts on that same GLP-1 receptor [NEJM, Sep 29, 2026]. Lilly is testing both drugs in one large liver disease study, but each is compared with placebo, not combined or compared with the other [ClinicalTrials.gov, NCT07165028].