Mounjaro Linked to 2% Greater Lean Mass Loss Than Ozempic: What the Latest Study Means for You

A newly published preprint study is adding an important nuance to the conversation around GLP-1 receptor agonists: while tirzepatide (brand names Mounjaro and Zepbound) often delivers more dramatic weight loss than semaglutide (Ozempic, Wegovy), it may come at the cost of a slightly greater loss of lean body mass — the muscle, connective tissue, and other non-fat components essential for metabolism, strength, and long-term health. The findings, which have not yet been peer-reviewed, are drawn from one of the largest real-world comparisons of these two powerhouse medications to date, and they arrive at a moment when millions of people are using them for chronic weight management.

Background: The GLP-1 Revolution and the Muscle Question

GLP-1 receptor agonists have reshaped obesity medicine by delivering weight loss that often exceeds 15% of body weight — a threshold previously seen only with bariatric surgery. Semaglutide and tirzepatide are the most widely prescribed agents in this class, but they work slightly differently. Semaglutide targets only the GLP-1 receptor, while tirzepatide is a dual agonist that also activates GIP receptors, making it a more potent metabolic intervention. In clinical trials, tirzepatide has consistently produced larger reductions in body weight than semaglutide, but head-to-head data on the composition of that weight loss have been scarce.

Lean body mass loss during weight reduction is a well-known physiological phenomenon. Typically, about 20 to 30 percent of weight lost through diet and medication alone is lean tissue rather than fat. The concern is that excessive muscle loss can lower resting metabolic rate, reduce physical function, and make long-term weight maintenance harder. Until this study, however, there was no large-scale, real-world analysis directly comparing how semaglutide and tirzepatide affect lean mass.

The Evidence: A Preprint Study of Nearly 8,000 Individuals

The study was posted on the preprint server medRxiv in April 2026 and has not yet undergone peer review. The researchers did not disclose the lead author’s name or institutional affiliation in the preprint; only the digital object identifier (DOI) was provided. The analysis included 7,965 adults who were newly prescribed either semaglutide (6,196 individuals) or tirzepatide (1,769 individuals). All participants were tracked from the start of treatment, with lean body mass measured before and after initiation of their GLP-1 medication. The observation period extended up to 12 months.

At the three-month mark, people taking tirzepatide had lost about 1.1% more lean body mass than those on semaglutide. By 12 months, that gap widened to 2.0%. In absolute terms, the difference is modest, but it could be clinically meaningful for people who are already at risk of muscle loss due to age, sedentary lifestyle, or coexisting conditions. The study authors also reported that roughly 10% of tirzepatide users achieved a 20% or greater total weight reduction alongside a 5% or greater reduction in lean mass. Among semaglutide users, only about 7% reached this combined threshold.

Several factors were associated with a greater lean mass loss risk: higher doses of medication, longer treatment duration, lower tolerance for physical activity during treatment, and pre-existing musculoskeletal conditions. The study was observational in nature, meaning it can show associations but cannot prove that tirzepatide directly causes greater lean mass loss compared to semaglutide. The lack of randomization and reliance on real-world prescribing patterns may introduce confounding variables, such as differences in baseline health status or motivation for weight loss.

What This Means for You: Muscle Protection Strategies Are Non-Negotiable

If you are taking a GLP-1 medication, or considering one, the key takeaway is not to abandon a more effective drug but to double down on muscle-preserving habits. Both semaglutide and tirzepatide lead to some degree of lean mass loss, and the difference between them — while statistically detectable — is small. What matters far more is what you do alongside the medication. Experts unanimously agree on two foundational pillars: progressive resistance training and consistently hitting protein targets.

Jeffrey Lee, MD, a double board certified plastic surgeon and GLP-1 prescriber who was not involved in the study, emphasized that “resistance training signals the body to maintain muscle, even in a calorie deficit.” He recommends making strength training a non-negotiable part of any weight loss plan that uses GLP-1 drugs. For those who struggle with low energy or poor tolerance for exercise during treatment, starting with short, low-intensity resistance sessions and gradually increasing volume may help bridge the gap. Alongside exercise, Mir Ali, MD, a bariatric surgeon and medical director of MemorialCare Surgical Weight Loss Center, stressed that adequate protein intake — typically at least 1.2 to 1.6 grams per kilogram of body weight per day for people actively losing weight — is essential. He also cautioned that the pace of weight loss matters: “Slower, more gradual weight reduction tends to better preserve lean mass compared to rapid weight loss. This is why careful dose titration and ongoing monitoring are key when using GLP-1s.”

Expert Perspective: Interpreting the Findings with Caution

Both Lee and Ali, who reviewed the preprint for this article, noted that the study’s results are directionally consistent with what they observe in clinical practice. “Tirzepatide is generally more potent than semaglutide, and with greater overall weight loss, you often see a higher degree of lean mass loss as well,” Lee said. However, they also pointed out important limitations. Because the study is a preprint, it has not been vetted by independent peer reviewers, and key methodological details — such as how lean body mass was measured (e.g., bioelectrical impedance vs. DXA), whether physical activity and dietary intake were controlled for, and what demographic factors were adjusted — are not yet fully transparent. Future peer-reviewed studies, especially randomized controlled trials that directly compare the two medications with precise body composition outcomes, will be needed to confirm these findings and to determine whether the 2% gap has any meaningful impact on long-term health, resting energy expenditure, or quality of life.

Frequently Asked Questions

Q: Does Mounjaro really cause more muscle loss than Ozempic?

Yes, the available real-world data suggest that people taking tirzepatide (Mounjaro) lose about 2% more lean body mass over 12 months compared to those on semaglutide (Ozempic). This difference is largely attributed to tirzepatide’s greater overall weight loss, not a direct muscle-wasting effect. However, the study is a preprint and has not yet been peer-reviewed.

Q: Why is lean body mass loss a concern during weight loss?

Lean body mass includes skeletal muscle, which is critical for metabolic health, physical strength, and mobility. Losing too much muscle can lower your resting metabolic rate, making it harder to sustain weight loss, and can increase the risk of fatigue, falls, and injury. Preserving muscle helps ensure that the weight you lose is predominantly fat.

Q: Can I prevent muscle loss while using GLP-1 medications?

Yes. Experts recommend a combination of regular resistance (strength) training at least two to three times per week and a daily protein intake of roughly 1.2 to 1.6 grams per kilogram of body weight. Slowing the pace of weight loss through careful dose titration also helps shift the composition of lost weight toward fat rather than muscle. Aerobic exercise is beneficial for cardiovascular health, but resistance training is the primary signal to preserve muscle in a calorie deficit.

Q: Should I switch from tirzepatide to semaglutide because of this study?

Not necessarily. The difference in lean mass loss is small, and any switch in medication should be based on a comprehensive evaluation with your healthcare provider, including factors like total weight loss achieved, side effects, cost, and individual health goals. The focus should be on adding muscle-preserving strategies rather than abandoning a more effective medication if it is otherwise working well.

Q: How much protein do I need to protect muscle on these medications?

While individual needs vary, a general target for active weight loss is 1.2 to 1.6 grams of protein per kilogram of body weight per day. For a person weighing 200 pounds (about 91 kg), that translates to roughly 109 to 145 grams of protein daily. Spreading protein evenly across meals and including a source of high-quality protein after resistance exercise can further support muscle preservation.

Sources

• No authors listed (2026). Comparative analysis of lean body mass changes in adults initiating semaglutide versus tirzepatide: a real-world study. medRxiv. DOI: 10.64898/2026.04.11.26350687v1 (preprint, not yet peer-reviewed).