Important: Retatrutide is investigational and not FDA-approved. Cross-trial comparisons are presented for educational purposes only. No head-to-head trials exist comparing all three medications. Published data come from separate studies with different designs, populations, and endpoints.
Weight Loss
RETATRUTIDE VS SEMAGLUTIDE VS TIRZEPATIDE
Three GLP-1 medications, three different mechanisms. Here's how they compare based on published clinical data.
Medically reviewed by Missy Zammichieli, DNP, APRN, FNP-BC · Updated March 25, 2026
THE FULL COMPARISON
| Attribute | Semaglutide | Tirzepatide | Retatrutide |
|---|---|---|---|
| Brand Names | Ozempic, Wegovy, Rybelsus | Mounjaro, Zepbound | TBD (investigational) |
| Mechanism | GLP-1 receptor agonist | GLP-1 + GIP dual agonist | GLP-1 + GIP + glucagon triple agonist |
| Receptors Targeted | 1 | 2 | 3 |
| Avg Weight Loss | ~15-17% (STEP trials, 68 wks) | ~20-22.5% (SURMOUNT, 72 wks) | ~24.2% (Phase 2, 48 wks) / ~28.7% (TRIUMPH-4, 68 wks) |
| Key Trials | STEP 1-5 | SURMOUNT 1-4 | Phase 2 (NCT04881706), TRIUMPH 1-4 |
| FDA Status | Approved (2017 / 2021) | Approved (2022 / 2023) | Phase 3 (expected 2026-2027) |
| Injection Frequency | Weekly subcutaneous | Weekly subcutaneous | Weekly subcutaneous (in trials) |
| Manufacturer | Novo Nordisk | Eli Lilly | Eli Lilly |
| Liver Fat Reduction | Modest reduction | ~50% relative reduction | ~86% relative reduction (Phase 2) |
| Muscle / Lean Mass | 25-40% of weight lost is lean mass | Possibly better preservation than semaglutide | Glucagon may improve body composition — data pending |
| CV Outcomes | SELECT trial: 20% MACE reduction | SURPASS-CVOT ongoing | No CVOT yet |
| GI Side Effects | Nausea 44%, vomiting 24% | Nausea 26%, vomiting 12% | Nausea 25-45%, vomiting 9-18% |
| Oral Option | Yes (Rybelsus) | No | No |
* All weight loss percentages are from the highest dose arms of their respective trials. Cross-trial comparison is approximate due to differences in study design, duration, and populations.
HOW THEY WORK: 1 VS 2 VS 3 RECEPTORS
All three medications mimic gut hormones your body produces naturally after eating. The difference is how many hormones they mimic and what each one adds to the equation.
Semaglutide
GLP-1 Agonist
Mimics GLP-1 (glucagon-like peptide-1), a hormone released by your gut after eating.
- 1. Signals fullness to the brain — reduces appetite and food noise
- 2. Slows gastric emptying — food stays in your stomach longer
- 3. Improves insulin sensitivity — better blood sugar control
The original. Proven effective, well-studied, and the only one with a completed cardiovascular outcomes trial showing benefit.
Tirzepatide
GLP-1 + GIP Dual Agonist
Does everything semaglutide does, plus mimics GIP (glucose-dependent insulinotropic polypeptide).
- + GIP enhances insulin secretion — additional blood sugar control beyond GLP-1 alone
- + GIP affects fat metabolism — may promote fat burning and reduce fat storage
Two keys instead of one. The dual mechanism tends to produce more weight loss than semaglutide alone and may have a milder GI side effect profile.
Retatrutide
GLP-1 + GIP + Glucagon Triple Agonist
Does everything tirzepatide does, plus activates the glucagon receptor. This is the new addition.
- + Glucagon increases energy expenditure — your body burns more calories at rest
- + Glucagon promotes fat oxidation — signals the body to use fat as fuel
- + Glucagon reduces liver fat — drives dramatic hepatic fat clearance
Three keys. The glucagon component is what separates retatrutide — it doesn't just reduce appetite, it actively increases the rate at which your body burns stored fat, particularly in the liver.
What this means practically: Each generation adds a receptor, and the weight loss numbers go up. But more receptors also means more biological complexity and less long-term safety data. Semaglutide has years of real-world evidence. Tirzepatide has growing evidence. Retatrutide is still in trials.
WEIGHT LOSS: WHAT THE TRIALS SHOW
Cross-trial comparison caveat: The numbers below come from separate clinical trials with different study designs, patient populations, baseline BMIs, lifestyle interventions, and durations. Directly comparing percentages across trials is inherently unreliable. The only way to definitively compare two medications is a head-to-head randomized trial. No such trial exists for all three.
Semaglutide -- STEP Trials
The STEP program enrolled over 4,500 participants across multiple trials studying semaglutide 2.4 mg for obesity.
-14.9%
Avg body weight loss (STEP 1, 68 wks)
-17.4%
With intensive behavioral therapy (STEP 3)
83%
Achieved at least 5% weight loss
Tirzepatide -- SURMOUNT Trials
The SURMOUNT program studied tirzepatide for weight management in over 5,000 participants. The highest dose (15 mg) produced the strongest results.
-22.5%
Avg body weight loss at 15 mg (SURMOUNT-1, 72 wks)
-20.9%
At 10 mg dose (SURMOUNT-1)
96%
Achieved at least 5% weight loss
Retatrutide -- Phase 2 & TRIUMPH Trials
The Phase 2 trial (NCT04881706) published in 2023 showed unprecedented weight loss. The TRIUMPH Phase 3 program is ongoing with early data reported.
-24.2%
Avg body weight loss at highest dose (Phase 2, 48 wks)
-28.7%
TRIUMPH-4 preliminary data (68 wks)
100%
Achieved at least 5% weight loss (Phase 2, highest dose)
The pattern is clear but imperfect: Each additional receptor correlates with more weight loss in trials. But context matters. The retatrutide Phase 2 trial had 338 participants vs thousands in STEP and SURMOUNT. Phase 2 results often look better than Phase 3 because of smaller, more selected populations. The TRIUMPH Phase 3 data will give a clearer picture of real-world performance.
SIDE EFFECTS COMPARED
All three medications share the same core side effect profile because they all activate the GLP-1 receptor. The differences are in severity and frequency, driven by the additional receptors.
| Side Effect | Semaglutide | Tirzepatide | Retatrutide |
|---|---|---|---|
| Nausea | ~44% | ~26% | 25-45% (dose-dependent) |
| Vomiting | ~24% | ~12% | 9-18% (dose-dependent) |
| Diarrhea | ~30% | ~21% | ~22% |
| Constipation | ~24% | ~17% | ~14% |
| Decreased Appetite | ~20% | ~15% | ~10-20% |
| Discontinuation (GI) | ~7% | ~4-7% | ~6% (Phase 2) |
Rates from STEP 1 (semaglutide 2.4 mg), SURMOUNT-1 (tirzepatide 15 mg), and Phase 2 retatrutide (12 mg). Rates vary by dose level; highest dose arms shown for each.
Key takeaway: Tirzepatide tends to have the mildest GI profile relative to its weight loss efficacy. Semaglutide has the highest nausea and vomiting rates. Retatrutide's side effect rates are dose-dependent and fall between the other two at moderate doses, but can approach semaglutide-level nausea at the highest doses. Importantly, GI side effects for all three are worst during dose titration and typically improve with continued use.
LIVER FAT: RETATRUTIDE'S STANDOUT DATA POINT
Metabolic-associated steatotic liver disease (MASLD, formerly NAFLD) affects roughly 30% of the U.S. population. Excess liver fat drives insulin resistance, inflammation, and can progress to cirrhosis. All three GLP-1 medications reduce liver fat to some degree, but the differences are substantial.
Semaglutide
Modest
Reduction in hepatic fat content. Meaningful but not the primary driver of the mechanism.
Tirzepatide
~50%
Relative reduction in liver fat. GIP receptor activity contributes to improved hepatic metabolism.
Retatrutide
~86%
Relative reduction in liver fat at highest dose in Phase 2. Glucagon receptor activation directly drives hepatic fat oxidation.
Why this matters: The glucagon receptor is the differentiator. Glucagon's primary job in the body is to mobilize stored energy, and the liver is its main target organ. When retatrutide activates the glucagon receptor, it directly signals the liver to break down and burn stored fat. This is not just a downstream effect of weight loss; it is a direct pharmacological action on hepatic fat. For patients with significant liver fat, this is a meaningful distinction.
Caveat: The 86% reduction figure comes from a Phase 2 sub-study with a small sample size. Phase 3 data (TRIUMPH program) will determine whether this holds up in a larger, more diverse population. Tirzepatide's liver fat data is more robust, and semaglutide has the most real-world evidence overall.
BODY COMPOSITION: THE MUSCLE QUESTION
The biggest concern with aggressive weight loss from any method is losing muscle along with fat. Approximately 25-40% of weight lost on GLP-1 medications is lean mass. This is comparable to calorie restriction (25-30%) and bariatric surgery (25-35%). The question is whether any of these three medications is better at preserving muscle.
Semaglutide
The most studied. STEP trial data showed that roughly 40% of weight lost was lean mass at 68 weeks. This raised the muscle concern that now defines the GLP-1 conversation. However, this is comparable to any method of rapid weight loss without resistance training.
Tirzepatide
SURMOUNT data suggests the lean mass to fat mass loss ratio may be slightly better than semaglutide. The GIP receptor may play a role in nutrient partitioning. But the data is not conclusive enough to make a definitive claim. More body composition studies are needed.
Retatrutide
The glucagon receptor adds an interesting theoretical angle. Glucagon promotes fat oxidation, meaning the body preferentially burns fat for energy. If the body is burning more fat, it may theoretically spare more muscle. Phase 2 data hinted at favorable body composition changes, but detailed lean mass vs fat mass data from Phase 3 is not yet available. This is the most speculative claim of the three.
What Actually Preserves Muscle
Regardless of which medication you take, the evidence is clear that the following interventions matter far more than drug selection for muscle preservation:
- Resistance training 2-4x per week — the single strongest signal to preserve lean mass during weight loss
- High protein intake (0.7-1.0 g per pound) — provides the building blocks muscles need to survive a calorie deficit
- Adequate total calorie intake — GLP-1s can suppress appetite so much that people undereat; extreme deficits accelerate muscle loss
- Body composition monitoring via DEXA scanning — the scale doesn't tell you what you're losing
CARDIOVASCULAR OUTCOMES
Weight loss alone improves cardiovascular risk factors. But the FDA increasingly wants to know whether these drugs reduce hard cardiovascular outcomes like heart attacks, strokes, and cardiovascular death (collectively called MACE: major adverse cardiovascular events). Here is where the three medications stand.
Semaglutide -- SELECT Trial (Completed)
The landmark SELECT trial (N=17,604) showed that semaglutide 2.4 mg reduced MACE by 20% compared to placebo in adults with overweight/obesity and established cardiovascular disease. This was a game-changer: it demonstrated cardiovascular benefit in people without diabetes, purely based on obesity and CV risk. Semaglutide is currently the only GLP-1 receptor agonist with a completed positive CVOT for an obesity indication.
Tirzepatide -- SURPASS-CVOT (Ongoing)
Tirzepatide's cardiovascular outcomes trial is ongoing and results are expected in the coming years. Surrogate markers from SURMOUNT and SURPASS trials are encouraging: significant improvements in blood pressure, triglycerides, HbA1c, and inflammatory markers. Most experts expect a positive result, but until the trial reports, the cardiovascular benefit is not confirmed for tirzepatide in an obesity-specific population.
Retatrutide -- No CVOT Yet
No cardiovascular outcomes trial has been initiated for retatrutide. The drug is still in Phase 3 for its primary obesity indication. CVOT data is likely years away. The glucagon receptor component theoretically has mixed cardiovascular implications: it improves lipid metabolism and liver fat but also raises heart rate in some patients. Until trial data exists, the cardiovascular profile of retatrutide remains unknown.
Bottom line: If cardiovascular risk reduction is a primary goal, semaglutide has the only proven data. Tirzepatide likely will too, but the data isn't in yet. Retatrutide is a question mark on CV outcomes for now.
COST AND AVAILABILITY
The practical reality: even the best medication doesn't help if you can't access it or afford it. Here is the current landscape.
Semaglutide
List price: ~$1,300-1,400/month (Wegovy for weight loss)
Insurance: Broadest coverage of the three. Many commercial plans now cover Wegovy for BMI 30+ (or 27+ with comorbidities). Medicare coverage expanded in 2026.
Availability: Widely available. Prior shortage issues have largely resolved.
Oral option: Rybelsus (oral semaglutide) is available but produces less weight loss than the injectable form.
Tirzepatide
List price: ~$1,000-1,100/month (Zepbound for weight loss)
Insurance: Growing coverage, though slightly less universal than semaglutide. Eli Lilly has been aggressive with savings programs.
Availability: Intermittent supply issues at certain dose levels, though improving.
Retatrutide
List price: Unknown. Expected to be priced at a premium to tirzepatide given the additional mechanism.
Insurance: Not applicable until FDA approval.
Availability: Only through clinical trial enrollment. No prescription access exists outside of trials.
Expected timeline: If Phase 3 results are positive, FDA filing could occur in late 2026 or 2027, with potential approval 6-12 months after that.
At Moonshot Medical: Our full Weight Loss program is $405/month and includes medication, medical oversight, labs, DEXA body composition tracking, and dose management. For patients whose insurance covers the GLP-1 medication, we offer a Medical Oversight program at $150/month. We currently prescribe semaglutide and tirzepatide. We will evaluate adding retatrutide to our formulary if and when it receives FDA approval.
WHO SHOULD CONSIDER WHICH?
There is no single "best" GLP-1 medication. The right choice depends on your situation, priorities, and what is currently accessible to you. Here is a decision framework.
Semaglutide may be the best fit if:
- • You want the medication with the longest track record and most real-world safety data
- • Cardiovascular risk reduction is a primary goal (SELECT trial data)
- • Your insurance covers Wegovy but not Zepbound
- • You prefer an oral option (Rybelsus), even though the injectable is more effective
- • You have tried tirzepatide and experienced intolerable side effects
Tirzepatide may be the best fit if:
- • You want stronger average weight loss than semaglutide with a potentially milder GI side effect profile
- • You have significant insulin resistance or type 2 diabetes (dual mechanism provides superior glycemic control)
- • Semaglutide was not effective enough at maximum dose
- • You want a currently available, FDA-approved option with the strongest weight loss data
Retatrutide may be worth watching if:
- • You have significant liver fat or MASLD and want the strongest hepatic fat reduction data
- • You are interested in maximum possible weight loss when it becomes available
- • You have not responded adequately to semaglutide or tirzepatide
- • You are comfortable waiting for FDA approval and Phase 3 data
- • You may be eligible for clinical trial enrollment (check ClinicalTrials.gov)
The honest answer:
For most people seeking weight loss today, the choice is between semaglutide and tirzepatide. Both are proven, FDA-approved, and available now. Retatrutide is the most promising pipeline drug but it is not accessible outside of clinical trials. Don't wait for the "perfect" drug when effective, proven options exist. You can always switch or add when new options become available.
COMMON QUESTIONS
Which GLP-1 medication is most effective for weight loss?
In clinical trials, retatrutide produced the highest average weight loss (24-29%), followed by tirzepatide (20-22.5%) and semaglutide (15-17%). However, these numbers come from separate trials and cannot be directly compared. Individual response varies significantly. Some patients respond better to semaglutide than tirzepatide, or vice versa. The most effective medication is the one that works for your specific biology with tolerable side effects.
Which has fewer side effects?
Tirzepatide tends to have lower nausea rates than semaglutide in trials (26% vs 44%), despite producing more weight loss. Retatrutide's GI profile varies by dose and falls between the two at moderate doses. All three cause similar types of side effects (nausea, vomiting, diarrhea) that typically improve over time. If one medication is intolerable, switching to another is a reasonable and common strategy.
Can I switch between them?
You can switch between semaglutide and tirzepatide with provider guidance. This is commonly done when one medication isn't producing adequate results or is causing intolerable side effects. Your provider will adjust dosing during the transition. Retatrutide is not yet available for prescription, so switching to or from retatrutide is not currently an option outside of clinical trials.
Which is best for preserving muscle mass?
No GLP-1 medication has been definitively proven better for muscle preservation. Retatrutide's glucagon activity may theoretically favor fat oxidation over muscle breakdown, and tirzepatide's GIP activity may support slightly better body composition than semaglutide, but neither claim has conclusive evidence. The interventions that actually preserve muscle are the same regardless of medication: strength training 2-4x per week, high protein intake (0.7-1g per pound of body weight), and DEXA-based body composition monitoring.
When will retatrutide be available?
Retatrutide is currently in Phase 3 clinical trials (the TRIUMPH program). If trials are successful, Eli Lilly could file for FDA approval in late 2026 or 2027, with potential market availability 6-12 months after filing. No specific launch date has been announced. Until then, it is only accessible through clinical trial enrollment. Semaglutide and tirzepatide are both FDA-approved and available now.
Does Moonshot Medical offer all three?
We currently prescribe semaglutide and tirzepatide as part of our medically supervised weight loss program. Retatrutide is not yet FDA-approved and cannot be prescribed. Once it receives approval, we will evaluate adding it to our formulary based on the full safety and efficacy data from Phase 3 trials.
References
- 1. Wilding JPH, et al. "Once-Weekly Semaglutide in Adults with Overweight or Obesity." N Engl J Med. 2021;384(11):989-1002. (STEP 1)
- 2. Wadden TA, et al. "Effect of Subcutaneous Semaglutide vs Placebo as an Adjunct to Intensive Behavioral Therapy on Body Weight." JAMA. 2021;325(14):1403-1413. (STEP 3)
- 3. Jastreboff AM, et al. "Tirzepatide Once Weekly for the Treatment of Obesity." N Engl J Med. 2022;387(3):205-216. (SURMOUNT-1)
- 4. Aronne LJ, et al. "Continued treatment with tirzepatide for maintenance of weight reduction." JAMA. 2024;331(1):38-48. (SURMOUNT-4)
- 5. Jastreboff AM, et al. "Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial." N Engl J Med. 2023;389(6):514-526. ClinicalTrials.gov: NCT04881706
- 6. Lincoff AM, et al. "Semaglutide and Cardiovascular Outcomes in Obesity without Diabetes." N Engl J Med. 2023;389(24):2221-2232. (SELECT)
- 7. Sanyal AJ, et al. "Triple hormone receptor agonist retatrutide for metabolic dysfunction-associated steatotic liver disease." Nat Med. 2024;30:2037-2048.
- 8. Coskun T, et al. "LY3437943, a novel triple glucagon, GIP, and GLP-1 receptor agonist for glycemic control and weight loss." Cell Metab. 2022;34(9):1234-1247.
- 9. Rosenstock J, et al. "Retatrutide, a GIP, GLP-1 and glucagon receptor agonist, for people with type 2 diabetes." Lancet. 2023;402(10401):529-544.
- 10. Heymsfield SB, et al. "Mechanisms, Pathophysiology, and Management of Obesity." N Engl J Med. 2017;376(3):254-266.
CURRENTLY AVAILABLE AT MOONSHOT
We prescribe semaglutide and tirzepatide today. Medical oversight, DEXA body composition tracking, labs, and dose management included.
We'll evaluate your health history, discuss which medication makes sense for your biology, and build a plan that protects muscle while you lose fat.
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