Mounjaro vs Wegovy: What the Clinical Trials Actually Show
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are weekly injectable medications that cause significant weight loss. Both have reshaped conversations about obesity treatment in the last few years. But they work differently at a molecular level, and the clinical trial data shows a meaningful difference in average outcomes. Here's what you actually need to know.
Different Drugs, Different Mechanisms
Wegovy contains semaglutide, which is a GLP-1 receptor agonist. It mimics a single gut hormone — glucagon-like peptide-1 — that regulates appetite, slows gastric emptying and (in people with diabetes) helps control blood sugar.
Mounjaro contains tirzepatide, made by Eli Lilly. Tirzepatide is a dual GIP and GLP-1 receptor agonist. It activates two hormonal pathways: GLP-1 (same as semaglutide) and GIP — glucose-dependent insulinotropic polypeptide. Activating both pathways appears to produce a more pronounced effect on appetite and weight than GLP-1 alone.
This dual mechanism is why researchers expected tirzepatide to outperform semaglutide from the outset — and the trial data has largely confirmed that expectation.
What the Trials Show
The key weight-loss trials are the STEP programme (semaglutide, which underpinned Wegovy's approval) and the SURMOUNT programme (tirzepatide, which underpinned Zepbound's approval — the weight-management version of Mounjaro).
STEP 1 (semaglutide 2.4mg, 68 weeks): average weight loss of approximately 14.9% of body weight. Around 86% of participants lost at least 5% of their body weight.
SURMOUNT-1 (tirzepatide at 5, 10 and 15mg, 72 weeks): average weight loss of 15%, 19.5% and 22.5% respectively at each dose level. About 63% of participants on the highest dose lost 20% or more of their body weight — a threshold rarely achieved with any prior pharmacological treatment.
The Head-to-Head Trial
The SURMOUNT-5 trial compared tirzepatide directly against semaglutide 2.4mg (Wegovy's maintenance dose) in people with obesity or overweight without type 2 diabetes. Results showed tirzepatide produced approximately 47% more relative weight loss than semaglutide. Participants on tirzepatide lost roughly 20.2% of their body weight versus 13.7% on semaglutide at 72 weeks.
This is the only head-to-head randomised controlled trial between these two drugs, and it's a significant result. But a few caveats are worth noting. Averages mask a wide range of individual responses. Some people respond exceptionally well to semaglutide. Some people on tirzepatide experience side effects that limit tolerability at the higher doses. The "better drug" on paper isn't always the better drug for a specific individual.
Mounjaro vs Zepbound: The Same Drug, Two Brands
This adds another layer of potential confusion. Mounjaro and Zepbound both contain tirzepatide. Mounjaro is approved for type 2 diabetes management. Zepbound is approved for chronic weight management. It's the same relationship as Ozempic and Wegovy — same active ingredient, different approval indications, sometimes different insurance coverage.
If you're being prescribed tirzepatide for weight loss, you're likely getting Zepbound (or Mounjaro off-label, depending on your country and insurer). The dosing is identical: starting at 2.5mg and escalating by 2.5mg every four weeks up to a maximum of 15mg.
Which One Should I Take?
Your prescriber's decision, not ours. But the practical factors typically come down to: what's available where you live, what your insurer will cover, what your medical history looks like, and how you respond to each medication. Tirzepatide appears to produce greater average weight loss based on current trial data. But access and cost are real barriers in many places, and individual response varies.
It's also worth noting that Wegovy has been on the market longer and has a larger body of real-world evidence supporting it. Tirzepatide is newer and still accumulating long-term data. Both are effective. Both are well-studied. Neither is a magic bullet — lifestyle changes alongside the medication remain the most important driver of long-term outcomes.
Track Wegovy, Mounjaro, Ozempic or Zepbound
The GLP-1 HQ tracker supports all four medications with their correct dose options. Log each injection, track your weight over time and see your progress toward your goal — free, private, and no download required.
Open the free tracker →What the Numbers Mean for You Personally
A 22% average weight loss sounds extraordinary — and it is, by the standards of obesity pharmacotherapy. But "average" means half of people did better, and half did worse. If you started at 100kg, a 22% loss would be 22kg. A 15% loss would be 15kg. Both are clinically significant. Both, for most people, would meaningfully change health outcomes and quality of life.
The best medication is the one you can access, afford, tolerate and stay on. Tracking your actual results over time — rather than comparing yourself to trial averages — is the most useful thing you can do to understand how your body is responding.
Medical disclaimer: Clinical trial data cited in this article refers to results from specific studies under controlled conditions and should not be taken as a guarantee of individual outcomes. This article is for informational purposes only and does not constitute medical advice. Always consult your prescriber before starting, stopping or changing medication.