Mounjaro vs. Ozempic: Study Finds One May Cause More Muscle Loss Than the Other

Understanding the GLP-1 Revolution and Its Hidden Trade-Offs

The rise of GLP-1 drugs has changed the way millions of people approach weight loss. Medications like Ozempic, Wegovy, Mounjaro, and Zepbound were originally developed to treat type 2 diabetes. But doctors soon noticed a powerful side effect: patients were losing significant amounts of weight. This discovery led to a surge in prescriptions for weight management.

Today, these drugs are among the most talked-about treatments in medicine. They work by mimicking hormones in your body that control appetite and blood sugar. When you take them, you feel fuller longer and eat less. For many people, this leads to dramatic weight loss. But as the new study shows, not all weight loss is created equal. The type of weight you lose matters just as much as the number on the scale.

How This Affects You as a Patient or User

If you are taking one of these medications or considering starting, this study has direct implications for your health. The key takeaway is that while tirzepatide (Mounjaro, Zepbound) may help you lose more weight overall, it also appears to take more muscle with it. Muscle is not just for athletes or bodybuilders. It is essential for everyday life.

Your muscles help you walk, climb stairs, carry groceries, and maintain your balance as you age. They also play a major role in your metabolism. Muscle tissue burns more calories at rest than fat tissue does. So when you lose muscle, your metabolism slows down. This can make it harder to keep weight off in the long run. It can also leave you feeling weaker and more tired than you expected.

For people who are already at risk for muscle loss — such as older adults, those with chronic illnesses, or people who are sedentary — the effects can be even more concerning. The study found that about 10% of people taking tirzepatide lost 5% of their lean body mass along with 20% of their total body weight. That is a significant amount of muscle to lose in a relatively short time.

What Experts Say About the Study and Its Limits

Medical experts who were not involved in the study agree that the findings make sense. Dr. Jeffrey Lee, a double board certified plastic surgeon who prescribes GLP-1s, pointed out that more potent weight loss drugs naturally cause more lean mass loss. “This finding is not entirely surprising,” he said. “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.”

However, experts also stress that this study is a preprint. That means it has not yet gone through the full peer-review process where independent scientists check the methods and conclusions. While the results are important and worth paying attention to, they are not the final word. More research is needed to confirm these findings and understand why the difference exists.

Dr. Mir Ali, a bariatric surgeon who was not involved in the study, added that the findings highlight something doctors have long suspected. “Both adequate protein intake and exercise are essential for preserving muscle,” he said. “This study reinforces that advice with real-world data.”

Why Muscle Loss Matters Beyond the Scale

When people focus only on the number on the scale, they may miss what is happening inside their bodies. Losing weight is good for many health conditions, including heart disease, diabetes, and joint pain. But if a large portion of that weight is muscle, the benefits can be reduced.

Muscle loss can lead to a condition called sarcopenia, which is the gradual loss of muscle mass and strength. This condition is especially dangerous for older adults because it increases the risk of falls and fractures. Even for younger people, losing too much muscle can cause fatigue, a slower metabolism, and a higher chance of regaining weight after stopping the medication.

There is also a cosmetic concern. When people lose muscle, their skin may appear looser or less toned. This is one reason why some people who lose a lot of weight with GLP-1 drugs seek plastic surgery afterward. Dr. Lee, who performs these surgeries, sees this regularly in his practice.

Who Is Most at Risk for Muscle Loss?

The study identified several factors that put people at higher risk. These include:

• Higher doses and longer time on the medication: The more medication you take and the longer you take it, the greater the chance of losing muscle.
• Low tolerance for exercise during treatment: People who cannot or do not exercise are more likely to lose lean mass.
• Existing musculoskeletal conditions: If you already have joint or muscle problems, you may be more vulnerable.

The study also found that about 7% of people taking semaglutide lost a significant amount of lean mass, compared to 10% of those on tirzepatide. This means that while the risk is higher with tirzepatide, it is not zero with semaglutide. Anyone using these drugs should be aware of the potential for muscle loss.

Practical Takeaways: How to Protect Your Muscle While Losing Weight

The good news is that muscle loss is not inevitable. There are steps you can take to protect your lean body mass while still getting the benefits of weight loss. Experts agree on three main strategies.

1. Prioritize Strength Training

Dr. Lee emphasized that resistance training signals your body to keep muscle, even when you are eating fewer calories. “Exercise (particularly strength training) plays a critical role in preserving lean muscle mass while on GLP-1 medications,” he said. You do not need to become a bodybuilder. Simple exercises like lifting weights, using resistance bands, or doing bodyweight moves like squats and push-ups can make a big difference. Aim for at least two to three sessions per week.

2. Eat Enough Protein

Protein is the building block of muscle. When you are eating fewer calories, it is even more important to make sure those calories come from high-quality sources. Dr. Ali stressed that “both aerobic exercise and resistance training are highly beneficial,” but he also noted that protein intake is just as crucial. Good sources include lean meats, eggs, dairy, beans, lentils, and protein shakes. Many experts recommend getting at least 1.2 to 1.5 grams of protein per kilogram of body weight per day while on these medications.

3. Lose Weight at a Steady Pace

Rapid weight loss is more likely to include muscle loss. Dr. Ali explained that “slower, more gradual weight reduction tends to better preserve lean mass compared to rapid weight loss.” This is why doctors carefully adjust your dose over time. “Careful dose titration and ongoing monitoring are key,” he added. If you are losing weight very quickly, talk to your doctor about whether your dose should be adjusted.

What This Means for Your Next Doctor’s Visit

If you are currently taking a GLP-1 drug or planning to start, this study gives you a reason to have a more detailed conversation with your healthcare provider. Ask about your specific risk for muscle loss. Discuss whether semaglutide or tirzepatide is the better choice for your body and your goals. And most importantly, ask for a plan that includes exercise and nutrition, not just a prescription.

Your doctor can help you monitor your body composition — not just your weight. Some clinics offer body scans that measure muscle mass versus fat mass. This can give you a clearer picture of what is happening inside your body. Tracking how your clothes fit, how strong you feel, and your energy levels can also provide useful clues.

The Bottom Line for Readers

GLP-1 drugs are powerful tools for weight loss, but they are not magic. They come with trade-offs, and muscle loss is one of the most important ones to understand. The new study suggests that tirzepatide (Mounjaro, Zepbound) may cause more muscle loss than semaglutide (Ozempic, Wegovy), especially over longer periods. But both drugs carry some risk.

The key is to be proactive. Do not rely on the medication alone. Combine it with strength training, adequate protein, and a steady pace of weight loss. By doing so, you can maximize the benefits of these drugs while protecting the muscle that keeps you strong, healthy, and mobile for years to come.

As Dr. Lee put it, “The more aggressive the weight loss, the greater the likelihood that some of that loss includes muscle, not just fat.” Knowing this gives you the power to take action. Talk to your doctor, make a plan, and take care of your body from the inside out.