Weight loss: the numbers

The data comes from two landmark trials.

Mounjaro (tirzepatide): The SURMOUNT-1 trial enrolled 2,539 adults with obesity. At the highest dose (15mg), participants lost an average of 22.5% of their body weight over 72 weeks (PMID: 35658024).

Retatrutide: Phase 3 data at the 12mg dose showed 28.7% average body weight loss. Earlier Phase 2 data published in the New England Journal of Medicine had already shown 24.2% weight loss at 48 weeks at the highest dose (PMID: 37385275). The Phase 3 results exceeded even those numbers.

Metric	Mounjaro (15mg)	Retatrutide (12mg)
Average weight loss	22.5%	28.7%
Trial duration	72 weeks	48 weeks (Phase 2) / ongoing (Phase 3)
Mechanism	GLP-1 + GIP	GLP-1 + GIP + Glucagon
FDA status	Approved	Investigational

A ~6 percentage point gap is significant. For someone weighing 250 pounds, that is the difference between losing roughly 56 pounds (Mounjaro) and 72 pounds (retatrutide). About 16 extra pounds of weight loss.

For more context on retatrutide's clinical data, see our full breakdown on retatrutide for weight loss.

The glucagon advantage

The glucagon receptor is what separates retatrutide from every other obesity drug on the market. Here is what it appears to do based on published data.

Increased energy expenditure. GLP-1 and GIP primarily reduce appetite. Glucagon receptor activation goes further by increasing how many calories your body burns. This creates a dual mechanism for weight loss: eating less AND burning more.

Liver fat reduction. Phase 2 data showed retatrutide reduced liver fat by over 80% in participants with MASLD (metabolic-associated steatotic liver disease). This is far beyond what tirzepatide or semaglutide have shown. Liver fat is a major driver of metabolic disease, so this has implications beyond just the number on the scale.

Fat oxidation. Glucagon promotes the breakdown of stored fat for energy. This may explain why retatrutide's weight loss skews more toward fat mass rather than lean mass, though more data is needed to confirm this.

Side effects compared

Both drugs share similar GI side effect profiles. Nausea, vomiting, diarrhea, and constipation are the most common complaints with both. These tend to be worst during the dose titration period and improve over time.

Side Effect	Mounjaro	Retatrutide
Nausea	Common	Common
Diarrhea	Common	Common
Vomiting	Common	Common
Constipation	Common	Common
Dysesthesia	Not reported	Reported in trials

The notable difference: retatrutide has a dysesthesia issue. Some trial participants reported unusual skin sensations (tingling, numbness, or warmth). This side effect is not seen with Mounjaro and may be related to glucagon receptor activation. The long-term significance is still being studied.

Safety track record. Mounjaro has years of real-world safety data from millions of prescriptions. Retatrutide has only been studied in controlled clinical trials with thousands of participants. This is a meaningful gap. Real-world data often reveals things trials do not catch.

For a broader look at GI side effects across this drug class, see our guide on GLP-1 side effects.

Availability and cost

This is where the practical differences hit hardest.

Mounjaro/Zepbound: Available now by prescription in the United States. Mounjaro is indicated for type 2 diabetes. Zepbound is the same drug approved specifically for weight management. List price runs approximately $1,000 to $1,100 per month without insurance. Manufacturer savings programs and insurance coverage can reduce this significantly.

Retatrutide: Not available outside of clinical trials. Phase 3 trials are ongoing. If results hold and FDA review goes smoothly, approval could come around 2027. Pricing has not been announced, though Eli Lilly has historically priced competitively within the obesity drug market.

The Eli Lilly strategy. Lilly will likely position retatrutide as the premium option above Mounjaro and Zepbound. They are not going to cannibalize their existing products. Expect retatrutide to be marketed for patients who need greater weight loss or who have specific metabolic concerns (like significant liver fat) that the triple mechanism addresses better.

For the latest on retatrutide's regulatory timeline, see our retatrutide FDA approval tracker.

Who should consider which?

Mounjaro/Zepbound makes sense if:

• You want a proven, FDA-approved option available today
• Your primary goal is weight loss and you want a well-established safety profile
• You prefer a drug with years of real-world data behind it
• Your insurance covers it or you can access manufacturer savings

Retatrutide may be worth watching if:

• You have significant liver fat or metabolic concerns beyond weight
• You have already tried GLP-1 or dual agonist medications without adequate results
• You are willing to wait for FDA approval and are not in urgent need of treatment now
• You want the highest weight loss potential based on current clinical data

For a comparison of tirzepatide for weight loss results across different trials, we cover that in a separate guide.

Frequently asked questions

Is retatrutide better than Mounjaro? Based on clinical trial data, retatrutide produces greater average weight loss (28.7% vs 22.5%). But "better" depends on your situation. Mounjaro is available now, FDA-approved, and has a much longer real-world safety track record. Retatrutide is still investigational.

Can I switch from Mounjaro to retatrutide? Not yet. Retatrutide is only available through clinical trials. If and when it receives FDA approval, switching would be a conversation between you and your doctor.

Are they the same drug? No. They are different molecules made by the same company (Eli Lilly). Mounjaro is tirzepatide, a dual agonist. Retatrutide is a triple agonist with an additional glucagon receptor component.

When will retatrutide be available? Phase 3 trials are ongoing. If results are positive and FDA review proceeds on schedule, approval could come around 2027. This timeline could shift in either direction.

Will retatrutide cost more than Mounjaro? Pricing has not been announced. Given that Eli Lilly will likely position it as a next-generation option, it may carry a premium. But Lilly has shown willingness to compete on price in the obesity space.

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Medical Disclaimer: The information on this website is for educational and informational purposes only. It is not intended as medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before starting any peptide protocol, medication, or supplement regimen. Individual results vary. We share published research, not medical recommendations.