Retatrutide vs Tirzepatide: The Ultimate 2026 Comparison

Retatrutide and tirzepatide both come from Eli Lilly, and they share most of their design. Each is a 39-amino-acid peptide built on the sequence of GIP, a gut hormone, with a fatty-acid chain that lets it ride on albumin in the blood and last a week between injections. Both switch on the GIP and GLP-1 receptors. Retatrutide adds a third, the glucagon receptor, and that single difference runs through every number that follows.

Their status is different. Tirzepatide has been an approved drug since May 2022, sold as Mounjaro for type 2 diabetes and, since November 2023, as Zepbound for weight loss. Retatrutide is still investigational, but the past year filled in most of the picture: five Phase 3 trials reported between December 2025 and July 2026, all of them positive, and Lilly expects to submit it to the FDA early in 2027. The first trial to test the two directly against each other, TRIUMPH-5, is underway.

Below, the two are compared on those Phase 3 results: how each one works, weight loss dose by dose, side effects, muscle and liver fat, the dosing schedules with a units chart, cost per week, and what happens when someone moves from tirzepatide to retatrutide.

Retatrutide vs tirzepatide at a glance

The fixed facts first, before any trial result:

Both weight-loss figures in the table are the results for people who stayed on treatment, which is the number each trial program leads with. The same measure is used for every trial comparison below.

What the third receptor changes

GLP-1 is the appetite receptor. Switching it on slows the stomach, turns hunger down in the brain and releases insulin when blood sugar is high. GIP also releases insulin, acts directly on fat tissue and appears to soften some of the nausea that GLP-1 causes. Tirzepatide matches natural GIP at the GIP receptor and binds the GLP-1 receptor about five times more weakly than natural GLP-1 does, so it is a GIP-heavy molecule.

Retatrutide leans even further toward GIP, where it is 8.9 times as potent as the natural hormone, and it is weaker at the other two receptors: 0.4 times natural GLP-1 and 0.3 times natural glucagon. Glucagon’s usual job is to raise blood sugar between meals, but its receptor also tells the liver to burn fat and raises how much energy the body spends. Inside retatrutide, the GLP-1 and GIP activity holds blood sugar down while the glucagon activity adds the energy side. In obese mice, retatrutide produced more weight loss than tirzepatide, and the extra came from higher energy expenditure.

Put simply, tirzepatide works mainly on how much is eaten. Retatrutide works on that too, and adds a push on how much is burned, most visibly in the liver.

Weight loss: the Phase 3 results

No trial has reported on the two side by side yet, so the fair comparison is trial against trial in the same kind of people. Lilly ran both programs the same way: weekly injections, dose steps every four weeks, a placebo group, and results for people who stayed on treatment.

Adults with obesity, without diabetes

At the top doses, retatrutide averaged 5.8 points more weight loss. Its trial ran eight weeks longer, but the gap doesn’t depend on that: retatrutide 12mg had already reached 24.2% by week 48 in its Phase 2 trial, more than tirzepatide 15mg reached in 72. On 12mg retatrutide, 45.3% of people lost 30% or more of their body weight and 27.2% lost 35% or more; 65.3% finished with a BMI under 30. On 15mg tirzepatide, 57% lost 20% or more.

Retatrutide also kept going. In a 532-person extension for people who started with a BMI of 35 or higher, those who stayed on 12mg averaged 30.3% weight loss at 104 weeks, or 85 lb.

Adults with obesity and type 2 diabetes

People with type 2 diabetes lose less weight on every drug in this class, and both compounds give up ground here. Retatrutide’s lead holds at 20.8% against 15.7% on the top doses. Tirzepatide posted the larger A1C drop, from a higher starting A1C.

Every retatrutide Phase 3 result so far

Tirzepatide’s record is longer. It has been through the SURPASS diabetes trials, the SURMOUNT obesity trials and outcome trials in sleep apnea and heart failure, and it has been prescribed to millions of people since 2022. Everything known about retatrutide comes from trials.

Is retatrutide stronger than tirzepatide?

A smaller number on the label doesn’t mean a weaker dose. Each molecule pulls differently at each receptor, so 12mg of retatrutide and 15mg of tirzepatide can’t be lined up milligram for milligram. The honest yardstick is the result each dose produced in its obesity trial. Sorted from least to most weight lost:

Three placements stand out. Retatrutide 4mg, one step above the 2mg start, beat tirzepatide 5mg and came within 2.4 points of tirzepatide 10mg. The whole top of tirzepatide’s range, 10mg and 15mg, fits in the gap between retatrutide 4mg and 9mg. And retatrutide’s two upper doses both finished above tirzepatide’s best. On trial results, retatrutide is the stronger compound.

TRIUMPH-5 will settle it head to head. About 800 adults with obesity are randomized to retatrutide or tirzepatide for 80 weeks, with neither they nor their doctors knowing which, and percent weight loss is the main result. The trial is scheduled to complete in November 2026.

Side effects compared

The two share one family of side effects: nausea, diarrhea, constipation and vomiting, mostly mild to moderate and mostly during the months of dose increases. Even on retatrutide’s top dose, about 89% of people stayed on it without stopping for side effects, and most never had nausea at all. Each compound’s rates sit next to its own trial’s placebo group, because placebo groups report these symptoms too.

Dose for dose, retatrutide 12mg brings more stomach side effects than tirzepatide 15mg, and more people stopped it (11.3% against 6.2%). The two top doses don’t deliver the same result, though: 28.3% weight loss against 22.5%. Matching doses by result instead of by rank changes the picture:

At similar weight loss, retatrutide 4mg added less nausea over placebo than tirzepatide 10mg, and fewer people stopped retatrutide 4mg for side effects than stopped placebo. Retatrutide’s higher rates at 12mg come with results tirzepatide never reached.

Dysesthesia: the side effect that sets retatrutide apart

Dysesthesia is an altered sense of touch: skin that tingles, burns, feels numb or reacts oddly to clothing and contact. It didn’t appear in retatrutide’s Phase 2 trial, then showed up in every Phase 3 trial, more often at higher doses:

Most people on retatrutide never reported it. Lilly described the cases as generally mild to moderate; most resolved while treatment continued, and most people who had it kept taking retatrutide. Tirzepatide shows the same effect at a small fraction of the rate.

Heart rate

Both raise resting heart rate, a class effect of GLP-1 drugs. On retatrutide in Phase 2, the rise was bigger at higher doses, topped out near week 24 and came back down over the rest of the trial.

Each of tirzepatide’s side effects is covered in the tirzepatide side effects guide, with separate articles on hair loss and fatigue.

Muscle loss: where the weight comes from

Both compounds take off mostly fat. In a SURMOUNT-1 body-scan study of 160 people, tirzepatide reduced body weight 21.3%, fat mass 33.9% and lean mass 10.9% over 72 weeks. About 75% of the weight lost was fat and 25% was lean mass, the same split seen in people who lost weight on placebo.

Retatrutide’s body-scan data comes from a Phase 2 study in type 2 diabetes. Over 36 weeks, fat mass fell 26.1% on 8mg and 23.2% on 12mg, against 4.5% on placebo, and the share of lean mass in the weight lost matched other obesity treatments. On both compounds the fat-to-lean split looks like ordinary weight loss. Because retatrutide takes off more weight in total, the lean mass lost adds up to more pounds.

Liver fat, blood sugar and heart markers

Liver fat is where the glucagon receptor shows most. In a Phase 2 study of 98 people with fatty liver disease, retatrutide 12mg cut liver fat by 82.4% at 24 weeks, while placebo left it where it was, and 86% of people on 12mg reached a normal liver fat level. Tirzepatide’s liver evidence comes from biopsies: in SYNERGY-NASH, the inflamed form of fatty liver disease (MASH) cleared without scarring getting worse in 62% of people on 15mg over 52 weeks, against 10% on placebo.

For blood sugar, retatrutide lowered A1C 1.7 to 2.0 points over 40 weeks in TRANSCEND-T2D-1, a trial in type 2 diabetes. In adults with severe obesity and heart disease (TRIUMPH-3), 12mg reduced triglycerides 37.0%, non-HDL cholesterol 16.5%, systolic blood pressure 9.3 mmHg and hsCRP, a marker of inflammation, 51.2%. Tirzepatide improved every heart and metabolic measure SURMOUNT-1 tracked, and it has finished outcome trials that retatrutide hasn’t reached yet. Retatrutide’s heart and kidney outcomes trial, TRIUMPH-Outcomes, enrolled about 10,000 people and is expected to finish in 2029.

Dosing schedules and units chart

Both trial programs start low and step up every four weeks, and the published results rest on these steps:

Retatrutide reaches its top dose four weeks sooner, and its 4mg group needed only one step. In the Phase 2 trial, starting at 2mg instead of 4mg reduced the stomach side effects, and all five Phase 3 trials began at 2mg.

Units on a U-100 syringe

Both come as a freeze-dried powder mixed with bacteriostatic water, and the number of units for a dose depends only on how much water goes in. At 10mg per mL (1mL of water for every 10mg in the vial), every 1mg is 10 units. At 20mg per mL (0.5mL of water for every 10mg), every 1mg is 5 units, which keeps the top doses inside a 1mL, 100-unit syringe.

The Simms Research peptide calculator works out any other combination of vial size, water and dose. The retatrutide dosage guide and tirzepatide dosage chart go through each schedule and vial size separately.

Retatrutide vs tirzepatide cost per week

Once water goes in, a vial has 28 days in the fridge, so the cheapest vial is the largest one that runs out inside that window, never more than five weekly doses. Both compounds come in 10mg, 20mg, 30mg and 60mg vials, and the sizes happen to match their trial steps in different ways. A tirzepatide vial holds exactly four doses at one step (10mg at 2.5mg, 20mg at 5mg, 30mg at 7.5mg, 60mg at 15mg); a retatrutide vial holds exactly five (10mg at 2mg, 20mg at 4mg, 30mg at 6mg, 60mg at 12mg). At Simms Research prices, that works out to:

Step for step, retatrutide costs within a few dollars a week of tirzepatide and delivered more weight loss. Retatrutide 4mg (19.0% average weight loss) comes to $26 a week, against $24 for tirzepatide 5mg (16.0%). At the top, retatrutide 12mg (28.3%) comes to $52 a week against $47 for tirzepatide 15mg (22.5%).

Retatrutide (GLP-3 RT) starts at $84 for a 10mg vial and Tirzepatide (GLP-2 TZ) at $65. Storage is the same for both, covered in Do peptides need to be refrigerated?

Switching from tirzepatide to retatrutide

No trial has moved people from one to the other. Every retatrutide Phase 3 trial started people at 2mg and excluded anyone who had used a weight-loss drug in the previous 90 days, so the published schedule was built for people starting fresh.

A common complaint after a switch is that hunger returns, and the ranking above shows why. Tirzepatide 15mg produced more weight loss than retatrutide 4mg, so a researcher who moves from the top of tirzepatide’s range to the bottom of retatrutide’s has lowered the effect, not held it. The effect catches back up as the retatrutide dose climbs. The glucagon receptor is also new territory for anyone arriving from tirzepatide, and dysesthesia rose with dose in every trial.

Tirzepatide leaves slowly. Its half-life is about five days, so roughly 38% of the last dose is still working seven days later and about 14% after fourteen days. The retatrutide dosage guide covers the first weeks of the schedule in detail.

Telling them apart on a certificate

In the vial, retatrutide and tirzepatide look identical: a small white cake of freeze-dried powder. Both are 39 amino acids long and share large stretches of sequence, so a purity percentage alone doesn’t say which one is inside. Mass does. Retatrutide weighs 4,731.3 g/mol and tirzepatide 4,813.5 g/mol, a gap mass spectrometry separates easily.

A certificate of analysis that includes an LC-MS identity result confirms the compound, and its net content line confirms how much is in the vial, which matters when every unit on the syringe is calculated from the label. Simms Research publishes the certificate for each batch of both compounds on its lab results page and on each product page.

Which is better: retatrutide or tirzepatide?

On weight loss, the evidence favors retatrutide. Its 9mg and 12mg doses beat tirzepatide’s top dose in every comparable trial, its 4mg dose came close to mid-dose tirzepatide with fewer people stopping for side effects, and people with a BMI of 35 or more kept losing weight to 30.3% at two years. Per week, it costs within a few dollars of tirzepatide for more weight loss.

Tirzepatide’s strengths are its record and a gentler top dose. It has been approved since 2022, has outcome data that retatrutide doesn’t have yet, and its 15mg dose brought less nausea and vomiting, fewer people stopping and almost none of the skin sensitivity seen on retatrutide 12mg.

Frequently asked questions

Is retatrutide stronger than tirzepatide?

Yes, by trial results. Retatrutide 12mg averaged 28.3% weight loss over 80 weeks in TRIUMPH-1, and tirzepatide 15mg averaged 22.5% over 72 weeks in SURMOUNT-1. Retatrutide 9mg (25.9%) also beat tirzepatide’s top dose.

What dose of retatrutide equals 15mg of tirzepatide?

There is no exact conversion. By average weight loss, tirzepatide 15mg (22.5%) falls between retatrutide 4mg (19.0%) and 9mg (25.9%).

Can you go from tirzepatide to retatrutide?

Many researchers do, but no trial has tested a switch. Retatrutide’s trials started everyone at 2mg after at least 90 days without weight-loss drugs, and a low retatrutide dose is weaker than high-dose tirzepatide, so appetite often returns during the climb.

Does retatrutide burn fat or muscle?

Mostly fat. In its body-scan study, fat mass fell 23–26% at 8mg and 12mg, and the share of lean mass in the weight lost matched other obesity treatments. On tirzepatide the split is about 75% fat and 25% lean mass.

Does retatrutide work immediately?

Weight loss builds over months. In Phase 2, retatrutide 12mg averaged 17.5% at 24 weeks and 24.2% at 48 weeks, and in TRIUMPH-1’s extension people were still losing weight at 104 weeks.

Which is safer, retatrutide or tirzepatide?

Tirzepatide has the longer safety record, with approval since 2022 and years of use outside trials. In trials the two share the same stomach side effects; retatrutide adds dysesthesia, and at 12mg more people stopped for side effects (11.3%) than on tirzepatide 15mg (6.2%).

How do they compare with semaglutide?

Semaglutide works on GLP-1 alone. Tirzepatide beat it head to head in SURMOUNT-5, 20.2% weight loss against 13.7% over 72 weeks at the highest dose each person tolerated, and retatrutide 12mg averaged 28.3% in TRIUMPH-1.

Can retatrutide and tirzepatide be taken together?

No trial has combined them. Both act on the GIP and GLP-1 receptors, so using both doubles up on the same targets.

When will retatrutide be approved?

Lilly’s FDA submission is scheduled for January to March 2027, backed by five positive Phase 3 trials. Approval would follow the FDA’s review.

Are retatrutide and tirzepatide for human use?

No. Simms Research sells both for laboratory research use only.