A new analysis of nearly 8,000 patients reveals that while tirzepatide produces greater overall weight loss, semaglutide may be gentler on muscle and lean body mass, suggesting the choice between these popular GLP-1 drugs involves more than just raw weight loss numbers.

Two of the most prescribed weight loss medications in America work differently at the cellular level, and a groundbreaking study published this week shows those differences matter for what you actually lose. Tirzepatide, sold as Zepbound and Mounjaro, and semaglutide, marketed as Wegovy and Ozempic, have become household names for their ability to help people shed pounds. But new research suggests they don't affect your body in identical ways .

Which Drug Preserves More Muscle During Weight Loss?

Researchers at nference, a Massachusetts-based data analytics firm, analyzed data from roughly 1,800 patients using tirzepatide and 6,200 using semaglutide to compare how much lean body mass, which includes muscle and connective tissue, each drug caused people to lose . The findings were striking: tirzepatide users consistently lost more muscle alongside their fat.

After three months of treatment, tirzepatide users lost an average of 1.1% more lean body mass than semaglutide users. By 12 months, that gap widened to 2% more lean body mass loss with tirzepatide . To put this in perspective, roughly 10% of tirzepatide users who lost more than 20% of their total body weight also lost more than 5% of their lean body mass. That same outcome occurred in fewer than 7% of semaglutide users with comparable weight loss .

The researchers tracked patients using low-radiation body composition scans or "smart" scales that measure body fat percentage, muscle mass, bone mass, and other components before and during treatment . This level of detail revealed a pattern that simple weight loss numbers alone would have missed.

Why Does This Matter for Women and People With Existing Health Conditions?

The muscle preservation difference becomes especially important for certain groups. The study found that people with a history of musculoskeletal pain before starting treatment faced a greater risk of losing lean body mass on both drugs, but the risk was higher with tirzepatide . Additionally, decreased exercise tolerance during treatment was linked with greater lean body mass loss in both groups, though again more pronounced in tirzepatide users .

This creates what researchers call a concerning cycle. If someone starts with a drug that carries a higher probability of muscle loss and already has joint or muscle problems, they become more likely to struggle with exercise. And when people don't exercise while taking these medications, they lose even more muscle mass .

How to Protect Muscle Mass While Using Weight Loss Medications

• Maintain Regular Exercise: Decreased exercise tolerance during treatment was directly linked to greater lean body mass loss, making physical activity a critical protective factor regardless of which medication you take.
• Consider Your Medical History: If you have a history of musculoskeletal pain or joint problems, discuss with your doctor whether semaglutide might be a better choice than tirzepatide, given the lower risk of muscle loss.
• Monitor Lean Body Mass, Not Just Weight: Ask your healthcare provider about body composition tracking using DEXA scans or bioelectrical impedance analysis rather than relying solely on scale weight.

The study cannot yet explain why tirzepatide causes greater lean body mass loss than semaglutide. Tirzepatide mimics two hormones, GLP-1 and GIP, while semaglutide mimics only GLP-1. Both hormones slow digestion and create feelings of fullness, but their different mechanisms may affect muscle preservation differently .

Higher doses and longer treatment duration were also associated with greater lean body mass decline in both drug groups, suggesting that the duration and intensity of treatment should factor into treatment planning .

"This suggests that patients shouldn't simplistically be thinking, 'I want to lose X amount of weight and I'll go with the option that delivers greater weight loss,'" said Venky Soundararajan, study leader at nference.

Venky Soundararajan, Study Leader, nference

Novo Nordisk, the manufacturer of semaglutide, noted that changes in muscle mass did not significantly differ between semaglutide and placebo groups in clinical trials, and that physical function was preserved in those studies . Eli Lilly, which makes tirzepatide, did not immediately respond to requests for comment on the new findings.

For people considering weight loss medications, especially those with existing joint or muscle concerns, this research adds an important dimension to the conversation with their healthcare provider. The goal isn't just to lose weight; it's to lose weight in a way that preserves the muscle and strength needed for long-term health and mobility.