Retatrutide and tirzepatide are two of the most talked-about peptides in weight loss research. Both are made by the same company, Eli Lilly, and both are given as a shot once a week. The big difference is where they stand today:
- Tirzepatide is already sold as a prescription medicine under the brand names Mounjaro and Zepbound.
- Retatrutide is still being tested in clinical trials and is not available as a medicine.
In this guide, you will learn how each one works, what the latest 2026 trial results show, how their side effects compare, and why a true side-by-side answer is still on the way. If you are new to this group of compounds, it helps to first understand how weight loss peptides work.
Retatrutide vs Tirzepatide at a Glance
The main difference is simple: tirzepatide acts on two hormone targets, while retatrutide acts on three.
| Feature | Tirzepatide | Retatrutide |
| Hormone targets | 2 (GLP-1 and GIP) | 3 (GLP-1, GIP, and glucagon) |
| How it is given | Weekly injection | Weekly injection |
| Current status | Approved prescription medicine | Still in clinical trials |
| Top average weight loss in its main obesity trial | 22.5% (72 weeks) | 28.3% (80 weeks) |
| Side effect seen more with it | None unique; mainly stomach-related | Dysesthesia (unusual skin sensations) |
| Head-to-head trial results | Not yet available | Not yet available |
The weight loss numbers come from separate trials, so they are a rough guide, not a direct match. The sections below explain why.
How Tirzepatide Works (Dual Action)
Tirzepatide copies two natural hormones that your gut releases after you eat:
- GLP-1 (glucagon-like peptide-1): tells your brain you are full, slows how fast food leaves your stomach, and helps your body release insulin when blood sugar rises.
- GIP (glucose-dependent insulinotropic polypeptide): also helps release insulin after meals and appears to boost the effects of GLP-1 on appetite.
Example: Think of hunger as a volume knob. GLP-1 turns the knob down. GIP helps turn it down a little further. With both signals working together, people tend to feel full sooner and eat less.
Because it targets two hormones, tirzepatide is often called a “dual agonist.” An agonist is simply a compound that switches on a receptor, much like a key turning a lock.
How Retatrutide Works (Triple Action)
Retatrutide acts on the same two targets as tirzepatide, GLP-1 and GIP, and adds a third one: glucagon.
- Glucagon is a hormone made by the pancreas. It normally raises blood sugar between meals, but it also signals the body to burn more stored energy and fat, especially in the liver.
By combining all three signals, retatrutide is called a “triple agonist.” Researchers believe the glucagon part is the main reason it may lead to more weight loss.
Example: If tirzepatide mainly helps you eat less, retatrutide aims to help you eat less and burn a bit more energy at the same time. It works on both sides of the energy balance.
| Action | Tirzepatide | Retatrutide |
| Reduces appetite | Yes | Yes |
| Helps insulin release after meals | Yes | Yes |
| May increase energy burning | Limited | Yes, through glucagon |
Retatrutide is sometimes called “GLP-3” online, but that is a nickname, not a scientific term. There is no hormone called GLP-3.
Weight Loss Results: What the Trials Show
In their main obesity trials, retatrutide led to more average weight loss than tirzepatide. Here are the top-dose results from each drug’s key studies:
| Trial | Drug and top dose | Length | Who took part | Average weight loss |
| TRIUMPH-1 (2026) | Retatrutide 12 mg | 80 weeks | Adults with obesity or overweight, no diabetes | 28.3% |
| TRIUMPH-4 (2025) | Retatrutide 12 mg | 68 weeks | Adults with obesity and knee arthritis | 28.7% |
| SURMOUNT-1 (2022) | Tirzepatide 15 mg | 72 weeks | Adults with obesity or overweight, no diabetes | 22.5% |
Later retatrutide trials in 2026 showed smaller but still large results in harder-to-treat groups:
- Obesity with type 2 diabetes (TRIUMPH-2):8% average weight loss at 12 mg
- Obesity with heart disease (TRIUMPH-3):6% average weight loss at 12 mg
To put this in real numbers: in TRIUMPH-1, people on the 12 mg dose started at about 248 pounds on average and lost about 70 pounds. In a longer extension for people with higher starting weight, average weight loss reached 30.3% at 104 weeks.
Why These Numbers Are Not a Direct Match
It is tempting to say retatrutide beats tirzepatide by about 6 points. But these results come from separate trials, and that makes a direct comparison unreliable:
- Different lengths: 80 weeks is longer than 72 weeks, and weight often keeps dropping over time.
- Different people: some trials included people with arthritis or diabetes, which can change results.
- Different time periods and sites: each trial had its own group of volunteers and study centers.
Example: Comparing two runners who ran on different days, on different tracks, and for different distances will not tell you who is faster. You need them to race side by side. That is exactly what the next trial is designed to do.
Is There a Head-to-Head Trial?
Yes, but the results are not out yet. A Phase 3 trial called TRIUMPH-5 is testing retatrutide directly against tirzepatide in adults with obesity.
| Detail | TRIUMPH-5 |
| What it compares | Retatrutide vs tirzepatide, side by side |
| Number of people | About 800 adults with obesity |
| Started | November 2024 |
| Expected to finish | Late 2026 |
| Results expected | Late 2026 or 2027 |
“Head-to-head” means both drugs are tested in the same trial, on the same type of people, over the same period. This removes most of the problems with comparing separate studies. Until TRIUMPH-5 reports, any claim that one drug is clearly better is based on indirect comparisons.
Side Effects Compared
Both peptides share the same common side effects, mostly in the stomach and gut. Retatrutide adds a few effects that appear to come from its glucagon action.
Shared side effects (seen with both):
- Nausea
- Diarrhea
- Vomiting
- Constipation
In trials, these effects were usually mild to moderate. They were most common while the dose was being slowly raised, and starting at a lower dose helped reduce them.
Effects seen more with retatrutide:
| Effect | What it means | What trials showed |
| Dysesthesia | An unusual skin feeling, such as tingling, burning, or sensitivity to touch | In TRIUMPH-1, reported by up to 12.5% of people on retatrutide vs 0.9% on placebo; mostly mild and often went away during treatment |
| Higher resting heart rate | The heart beats a few more times per minute at rest | In the Phase 2 trial, heart rate rose with higher doses, peaked around week 24, then eased off |
| Urinary tract infections | Infections of the bladder or urinary tract | Slightly more common than placebo (about 8% vs 5%) |
Example: A small rise in heart rate is a bit like the effect of a morning coffee. For most healthy people it may not matter, but researchers are watching it closely for people who already have heart problems.
Long-term safety data for retatrutide is still being collected, while tirzepatide has several years of wider use behind it.
Does Retatrutide Burn Fat or Muscle?
Mostly fat, but some muscle and other lean tissue is lost too. This is true for almost every method of fast, large weight loss, including tirzepatide.
The best data so far comes from a Phase 2 body composition study in 189 adults with type 2 diabetes. Researchers used a DXA scan, a type of low-dose X-ray that measures fat, muscle, and bone separately. After 36 weeks:
- Total fat mass dropped by about 26% on the 8 mg dose and 23% on the 12 mg dose.
- Fat loss was larger than lean tissue loss in every effective dose group.
- The researchers described the share of fat lost as similar to other obesity treatments.
What this means in simple terms: retatrutide did not appear to cause more muscle loss than expected for the amount of weight lost. But because people lose more total weight on it, the total amount of lean tissue lost can also be larger.
Example: If one person loses 20 pounds and another loses 30 pounds with the same fat-to-muscle ratio, the second person loses more muscle in pounds, even though the ratio is the same.
Body composition results from the larger Phase 3 trials are still pending. Protecting muscle during weight loss is also one reason interest in peptides for muscle growth keeps rising.
Current Status and Availability
The two peptides are at very different stages:
| Feature | Tirzepatide | Retatrutide |
| Status | Approved prescription medicine | Investigational (still in trials) |
| Brand names | Mounjaro, Zepbound | None yet |
| How people get it | Prescription from a doctor | Only through clinical trials as a medicine |
| Expected next step | Ongoing use and new trials | Company plans to file for approval after its Phase 3 trials |
You may also see retatrutide sold online as a “research peptide,” usually labeled “Research Use Only.” These products are meant for lab studies. They are not the same as the drug tested in clinical trials, and their quality can vary a lot from one seller to another.
Researchers looking for research-based peptides should choose a supplier that shares a full certificate of analysis (COA) for every batch. A COA is a lab report that confirms what is in the vial and how pure it is, and knowing how to read third-party tested peptides reports makes that check much easier.
Frequently Asked Questions
What is more powerful than tirzepatide?
In clinical trials so far, retatrutide has produced more average weight loss than tirzepatide did in its own trials (about 28% vs about 22.5% at top doses). However, the two have not yet been compared in the same trial, so this is not a final answer.
Can you switch from tirzepatide to retatrutide?
Retatrutide is not available as a prescription medicine, so there is no approved way to switch. In clinical trials, any change between treatments follows the study’s own safety rules under medical supervision.
Does retatrutide work immediately?
No. In trials, the dose was raised slowly, in steps about every four weeks, to help limit stomach side effects. Weight loss built up gradually over many months, with the largest results seen after 68 to 104 weeks.
Is retatrutide the same as GLP-3?
“GLP-3” is an online nickname for retatrutide. It is not a real hormone or scientific name. The name likely comes from retatrutide acting on three targets instead of one or two.
How do retatrutide, tirzepatide, and semaglutide compare?
They form a simple ladder by number of targets:
| Peptide | Targets | Top average weight loss in main trials |
| Semaglutide | 1 (GLP-1) | About 15% |
| Tirzepatide | 2 (GLP-1, GIP) | About 22.5% |
| Retatrutide | 3 (GLP-1, GIP, glucagon) | About 28% |
As with the main comparison, these results come from separate trials with different designs.
Final Thoughts
Retatrutide and tirzepatide share the same base design, but retatrutide adds a third target that may help the body burn more energy. Here is what to remember:
- Tirzepatide acts on two hormones (GLP-1 and GIP); retatrutide acts on three (adding glucagon).
- In separate trials, retatrutide showed about 28% average weight loss vs about 22.5% for tirzepatide.
- Both cause similar stomach side effects; retatrutide also showed skin sensations and a higher resting heart rate.
- Tirzepatide is an approved prescription medicine; retatrutide is still in trials.
- The first head-to-head results (TRIUMPH-5) are expected in late 2026 or 2027.
Disclaimer: This article is for informational and educational purposes only. It is not medical advice. Research peptides discussed here are not intended for human or veterinary use. Always speak with a qualified healthcare provider before making any health decision.
