Retatrutide vs Mounjaro: A1C, Weight and Side Effects in Type 2 Diabetes

Retatrutide vs Mounjaro is a close race on blood sugar and a clear win for retatrutide on weight, with one big catch: only Mounjaro is approved. Mounjaro is Lilly's brand of tirzepatide for type 2 diabetes. Retatrutide is a different Lilly molecule, still in Phase 3. In two 40-week trials of people with diabetes who started at the same 7.9% A1C, the biggest average A1C drop was 2.0% on retatrutide 9 mg (TRANSCEND-T2D-1) and 2.07% on Mounjaro 15 mg (SURPASS-1), while weight loss topped out at 16.6 kg against 9.5 kg. Each drug has also beaten semaglutide on A1C in its own head-to-head trial, by a similar 0.4 to 0.45 points.

No trial has put the two side by side in diabetes, so everything below compares separate studies, matched two ways: placebo trials with the same starting A1C, and each drug's trial against semaglutide.

Retatrutide vs Mounjaro at a glance

Sources: Lilly's TRANSCEND-T2D-1 release and paper; the SURPASS-1 paper and the Mounjaro label (revised August 2026).

Mounjaro has been approved in the US since May 2022. Retatrutide's first Phase 3 diabetes result came almost four years later, in March 2026, and its head-to-head with Ozempic on October 28, 2026, the sixth Phase 3 trial to report.

Lilly last said, on September 29, 2026, that it plans to submit retatrutide to the FDA in the first quarter of 2027 as a Biologics License Application (BLA). On October 2, 2026, the FDA decided that retatrutide is a drug, not a biological product, so an application would need to be filed under section 505 of the Federal Food, Drug, and Cosmetic Act. Lilly's appeal on that question is pending in the Seventh Circuit, and we found no ruling as of October 31, 2026.

Is retatrutide the same as Mounjaro?

No. Both are once-weekly injections from Eli Lilly, and both act on the GIP and GLP-1 receptors. Retatrutide also switches on the glucagon receptor, which is why Lilly calls it a triple agonist.

That third target is odd for a diabetes drug. Glucagon's normal job, as MedlinePlus puts it, is to make the liver release stored sugar into the blood, so you'd expect a glucagon-receptor drug to push blood sugar up. In the trials it didn't: A1C fell in every retatrutide arm of TRANSCEND-T2D-1, TRANSCEND-T2D-2 and TRIUMPH-2.

In the UK, the MHRA also authorized Mounjaro for weight management on November 8, 2023. In the US the weight brand is Zepbound, covered on retatrutide vs Zepbound; the molecule-level comparison lives on retatrutide vs tirzepatide.

A1C in two matched 40-week trials

TRANSCEND-T2D-1 and SURPASS-1 are about as close as two separate trials get. Both ran 40 weeks. Both enrolled adults with type 2 diabetes who were managing on diet and exercise alone, and both started at an average A1C of 7.9%.

Sources: Lilly's March and June 2026 releases and the TRANSCEND-T2D-1 paper; the SURPASS-1 abstract and Table 4 of the Mounjaro label.

Read the raw drops and it's a tie. Read the placebo row and it isn't. People on placebo in TRANSCEND-T2D-1 cut their A1C by 0.8 points on their own, while SURPASS-1's placebo group didn't move. Subtract placebo and Mounjaro's lead over doing nothing comes out roughly 0.5 to 1 point bigger, depending on which analysis you use. We couldn't find an explanation for why that placebo group improved so much.

Either way, the two land in the same place: about 9 in 10 people on the best-performing doses of either drug got their A1C under 7%.

Which beat semaglutide by more?

An active comparator gets around the placebo problem. Both drugs have been tested head to head against semaglutide, the drug in Ozempic, in adults with type 2 diabetes on metformin: Mounjaro in SURPASS-2, retatrutide in TRANSCEND-T2D-2.

Sources: the SURPASS-2 abstract and Table 5 of the Mounjaro label; Lilly's October 28, 2026 release, whose figures are all efficacy estimand (as if everyone stayed on treatment).

On blood sugar the two leads are almost the same, and so are the yardsticks: the semaglutide arms landed at -1.86% and -1.8%. The label's stricter look at SURPASS-2, which keeps in people who stopped or added drugs, still gives -2.3% against -1.9%. For retatrutide, Lilly says the 12 mg comparisons were controlled for chance findings across endpoints; the 9 mg A1C win was nominal only.

Weight is where they part again. Retatrutide's lead over semaglutide was more than twice Mounjaro's, but its trial ran twice as long, and weight was still falling at week 40 in TRANSCEND-T2D-1, so some of that gap may be time.

Our read: against the same yardstick, the two look about equal on A1C and retatrutide looks stronger on weight. The 40-week placebo trials say the same, so if blood sugar is your main problem, the published data give no reason to expect retatrutide to beat Mounjaro.

Weight loss in people with type 2 diabetes

Retatrutide pulled well ahead here. In TRANSCEND-T2D-1, people lost 11.1 kg on 4 mg, 15.1 kg on 9 mg and 16.6 kg on 12 mg over 40 weeks (11.5% to 16.8% of body weight, efficacy estimand). SURPASS-1 reports 7.0 to 9.5 kg on Mounjaro over the same 40 weeks. Even the lowest retatrutide dose beat Mounjaro's top dose by more than 1.5 kg.

The retatrutide group was heavier to begin with (average BMI 35.8 against 31.9), and the same percentage loss is more kilograms on a heavier body. That shrinks the gap but doesn't close it.

The longer trials in people with obesity and diabetes give a closer check. The TRIUMPH-2 paper in The Lancet counts everyone randomized, whether or not they stayed on the drug (the treatment-regimen estimand), and reports 18.8% average weight loss on retatrutide 12 mg at 80 weeks, against 5.1% on placebo. SURMOUNT-2, using the same kind of analysis, reported 14.7% on tirzepatide 15 mg at 72 weeks, against 3.2% on placebo.

Every trial's weight curve is on retatrutide weight loss results, and how much of the loss is muscle is on retatrutide and muscle loss.

Side effects, low blood sugar and dropouts

The stomach side effects are the same kind on both drugs. They're more frequent on retatrutide's top dose.

The dropout row counts different things: Lilly reports stopping for any adverse event, the label for stomach reactions only. If nausea is your worry, our page on retatrutide nausea covers how long it lasts and what helps.

Severe low blood sugar didn't show up on either drug used alone: none in the TRANSCEND-T2D-1 paper, and no readings below 54 mg/dL in SURPASS-1. Add insulin or a sulfonylurea and the Mounjaro label warns of a higher risk, including severe episodes, and says those doses may need to come down. Retatrutide has no insulin results yet. TRANSCEND-T2D-3, which adds it to basal insulin in people with kidney impairment, lists an estimated primary completion of October 2026, and Lilly hadn't announced results as of October 31, 2026.

Doses don't convert

Retatrutide 12 mg isn't a stronger version of Mounjaro 12.5 mg or 15 mg. They're different molecules, and no published table maps one onto the other.

Mounjaro's label climbs from 2.5 mg in 2.5 mg steps, at least 4 weeks apart, to 15 mg in adults and 10 mg in children. Retatrutide's diabetes trials went from 2 mg up every 4 weeks toward 4, 9 or 12 mg; the retatrutide dosage page lists every trial schedule.

Both are weekly shots because both stay in the body for days. The Mounjaro label puts tirzepatide's half-life at about 5 days, and Lilly Medical Information gives retatrutide's as about 6 days (retatrutide half-life has the detail).

No published trial has moved people from Mounjaro onto retatrutide, and none has tested the two together. Our guide to switching from tirzepatide to retatrutide sets out what's known, and retatrutide and tirzepatide together covers the overlap.

Heart outcomes, cost and access

Mounjaro now has heart data that retatrutide won't have for years. SURPASS-CVOT randomized 13,299 adults with type 2 diabetes and established heart or blood vessel disease to Mounjaro or dulaglutide (Trulicity). Over a median of 210 weeks, heart attack, stroke or heart death happened in 12.1% on Mounjaro and 13.0% on dulaglutide (hazard ratio 0.92, 95.3% CI 0.83 to 1.01). That met the trial's test for being no worse than dulaglutide, though superiority wasn't shown, and the label now lists lowering heart risk as an approved use.

Retatrutide's closest data point is TRIUMPH-3, an 80-week obesity trial in people with established heart or blood vessel disease. It recorded 27 heart attacks, strokes or heart deaths on retatrutide and 23 on placebo (hazard ratio 1.12, 95% CI 0.64 to 1.96). An interval that wide can't show benefit or harm. TRIUMPH-Outcomes, with about 10,000 people, has an estimated primary completion of February 2029.

Cost is simpler. As of October 2026, Lilly's savings card lets people with commercial insurance that covers Mounjaro pay as little as $25 for up to a 3-month fill, or as low as $499 for a 1-month fill if the plan doesn't cover it. The card ends on December 31, 2026 and excludes government plans such as Medicare and Medicaid.

Retatrutide has no price because there's no approved product, and Lilly's October 28, 2026 release repeats that it "cannot be legally sold or marketed for human use". Vials labeled as retatrutide still turn up online, and none is Lilly's product. If you're weighing listings of retatrutide for sale, that page explains what people are actually buying, and is retatrutide legal covers the law.

Which is better, retatrutide or Mounjaro?

For type 2 diabetes today, Mounjaro. Its A1C drops are about even with retatrutide's and its lead over semaglutide is about the same, it has a heart-risk indication, and a doctor can prescribe it. Retatrutide's real edge is weight: roughly 7 kg more lost in 40 weeks at the top doses, with more nausea and vomiting along the way.

Our read: the people with the strongest case to watch retatrutide are those whose diabetes is controlled but whose weight isn't, and no published retatrutide trial has studied people already taking Mounjaro. Questions worth taking to your doctor:

• Is my A1C at target on Mounjaro, and is weight now the bigger problem?
• Would a retatrutide trial take me? As of October 31, 2026, the only Lilly retatrutide trial still recruiting on ClinicalTrials.gov is SYNERGY-Outcomes, in fatty liver disease.
• If I take insulin or a sulfonylurea, how would either drug change those doses?
• Would my plan cover retatrutide after approval, or only tirzepatide?

The first head-to-head answer will come from TRIUMPH-5, which compares retatrutide with tirzepatide in about 800 adults with obesity and lists an estimated primary completion of November 2026. Our retatrutide clinical trials page tracks it.