Mounjaro and Ozempic are now the two most searched weight-loss injections among patients in Estonia and Finland. The question at almost every consultation is the same: which one produces better results?

The honest answer requires some context. No peer-reviewed randomised controlled trial has yet published a direct, prospective comparison of tirzepatide and semaglutide 2.4 mg in the same patient population. What exists are two large, separate phase 3 trials with different designs, plus two independent 2025 systematic reviews that constructed indirect statistical comparisons from those trials.

This article covers what each trial found, where the cross-trial comparison gets complicated, and what the best available evidence suggests when choosing between Mounjaro and Ozempic for weight management.

What are these two medicines?

Ozempic (semaglutide 1 mg injection) is approved for type 2 diabetes. Wegovy (semaglutide 2.4 mg injection) is the higher-dose version approved for chronic weight management. Patients and clinicians often refer to both collectively as "Ozempic." The weight-loss evidence reviewed here uses the 2.4 mg Wegovy dose throughout.

Mounjaro (tirzepatide) is a newer medicine from Eli Lilly that works differently. Semaglutide targets a single receptor: GLP-1 (glucagon-like peptide-1). Tirzepatide activates two receptors in parallel: GLP-1 and GIP (glucose-dependent insulinotropic polypeptide). Both receptors regulate appetite, gastric emptying, insulin release, and energy balance. The dual mechanism is why tirzepatide is sometimes called a twincretin.

Both medicines are weekly subcutaneous injections requiring dose titration over several months to reach the maintenance dose.

STEP 1: what Wegovy achieved in its phase 3 trial

The STEP 1 trial (Wilding et al., New England Journal of Medicine, 2021) is the pivotal phase 3 study for semaglutide 2.4 mg in obesity without diabetes. It enrolled 1,961 adults and followed them for 68 weeks, randomised to semaglutide 2.4 mg weekly or placebo, with lifestyle counselling for both groups.

Results at 68 weeks:

Mean weight loss, semaglutide 2.4 mg: 14.9% of body weight
Mean weight loss, placebo: 2.4%
86.4% of semaglutide participants lost at least 5% of body weight
69.1% lost at least 10%
50.5% lost at least 15%

Nausea was the most common side effect, affecting 44% of the semaglutide group versus 16% on placebo. Most events were mild to moderate and resolved after the titration phase.

SURMOUNT-1: what Mounjaro achieved in its phase 3 trial

The SURMOUNT-1 trial (Jastreboff et al., New England Journal of Medicine, 2022) is the pivotal phase 3 study for tirzepatide in obesity without diabetes. It enrolled 2,539 adults across three active dose arms (tirzepatide 5 mg, 10 mg, and 15 mg weekly) plus placebo, and followed participants for 72 weeks.

Results at 72 weeks:

Tirzepatide 15 mg: mean weight loss of 20.9% of body weight
Tirzepatide 10 mg: mean weight loss of 19.5%
Tirzepatide 5 mg: mean weight loss of 15.0%
Placebo: mean weight loss of 3.1%
57% of patients on tirzepatide 15 mg lost at least 20% of body weight

Nausea occurred in 31 to 45% of tirzepatide participants depending on dose, compared with 15% on placebo. As in STEP 1, most events were mild to moderate and clustered during dose escalation.

Important context: SURMOUNT-1 and STEP 1 enrolled different patient populations, ran at different times, and used different trial durations (72 vs 68 weeks). The placebo arms also had different outcomes (3.1% vs 2.4% weight loss), suggesting the baseline populations were not identical. Comparing the raw weight-loss percentages directly is therefore misleading.

Why you cannot simply compare the headline numbers

On first reading, 20.9% versus 14.9% appears to be a decisive advantage for Mounjaro. The comparison is genuinely more complicated.

SURMOUNT-1 ran for 72 weeks; STEP 1 for 68 weeks. Four additional weeks of treatment allows more weight loss to accumulate. The two placebo arms had different outcomes (3.1% versus 2.4%), a sign that the enrolled populations were not equivalent despite similar inclusion criteria. Age, baseline BMI, and behavioural factors may all have differed.

Critically, neither trial was designed to compare tirzepatide against semaglutide. Patients in each study were randomised against their own placebo control, not against the other drug. Without a shared randomised comparison arm, cross-trial differences could reflect population-level variation as much as true pharmacological differences.

What the 2025 indirect comparisons found

Two independent systematic reviews published in 2025 attempted to estimate what a direct comparison might show, using network meta-analysis methods to adjust for the different trial designs.

Singh et al. (Diabetes, Metabolic Syndrome and Obesity, 2025) conducted an indirect treatment comparison between SURMOUNT-1 and STEP 1 data. Their estimate: tirzepatide 10 mg and 15 mg were associated with approximately 4 to 5.4 additional percentage points of weight loss compared with semaglutide 2.4 mg. The review also noted fewer gastrointestinal adverse events with tirzepatide in the indirect comparison. The authors explicitly called for a direct randomised trial to confirm these findings.

Moiz et al. (Annals of Internal Medicine, 2025) conducted a separate systematic review and network meta-analysis. Their summary estimates: tirzepatide 15 mg was associated with 17.8% weight loss (95% confidence interval 16.3 to 19.3%) at 72 weeks; semaglutide 2.4 mg was associated with 13.9% weight loss (95% confidence interval 11.0 to 16.7%) at 68 weeks. At the time of analysis (search cutoff October 2024), no randomised head-to-head trials were available.

Indirect comparisons are useful for forming hypotheses, but they cannot fully control for all differences between trial populations. The confidence intervals reported by Moiz et al. overlap, and a statistically modelled difference does not always translate into a clinically meaningful difference for a given individual. Both reviews called for randomised evidence directly comparing the two drugs.

What happens when you stop either medicine

Both semaglutide and tirzepatide are maintenance treatments, not short courses. Stopping either medicine after successful weight loss leads to weight regain. This matters more for individual treatment selection than people often expect, because the question is not which medicine produces the fastest initial result but which one can be sustained safely over time.

The SURMOUNT-4 trial (Aronne et al., JAMA, 2024) randomised patients who had already lost weight on tirzepatide to either continue the medicine or switch to placebo for a further 52 weeks. Patients who continued tirzepatide reached a mean of 25.3% weight loss from their original baseline. Patients who stopped and received placebo regained a substantial portion of the weight they had lost. This pattern closely mirrors what was observed in the equivalent semaglutide maintenance study, where discontinuing Wegovy produced rapid weight regain over 12 months.

Long-term treatment continuity, tolerability, and access are therefore as important as peak efficacy figures when choosing between Mounjaro and Ozempic.

Body composition: what the evidence actually shows

Patients frequently ask whether Mounjaro preserves more muscle mass than Ozempic. This would be clinically important, since lean mass loss during weight reduction affects resting metabolic rate and long-term weight maintenance. The evidence on this point is limited and does not currently support superiority claims for either drug.

A 2025 body composition substudy of SURMOUNT-1 participants (Look et al., Diabetes, Obesity and Metabolism, 2025) measured the composition of weight lost with tirzepatide. Approximately 75% came from fat mass and approximately 25% from lean mass. This ratio is not exceptional relative to what is achievable with adequate dietary protein and resistance exercise.

No peer-reviewed randomised trial has directly compared body composition outcomes between tirzepatide and semaglutide 2.4 mg under controlled, identical conditions. Claims that Mounjaro is substantially superior to Ozempic at preserving muscle are not supported by current primary trial evidence. Both medicines carry a meaningful risk of lean mass reduction that requires active mitigation through nutrition strategy and progressive resistance training.

Side effects: is one medicine better tolerated?

Both medicines cause gastrointestinal side effects during dose escalation. Nausea, vomiting, and diarrhoea are the most common. In SURMOUNT-1, nausea affected 31 to 45% of tirzepatide participants depending on dose. In STEP 1, nausea affected 44% of semaglutide participants. These figures come from separate patient populations and cannot be compared directly.

The Singh et al. 2025 indirect comparison suggested fewer total gastrointestinal events with tirzepatide than with semaglutide 2.4 mg, but this finding needs confirmation from direct randomised evidence. Individual tolerance varies considerably. Some patients tolerate one medicine well and experience significant side effects from the other. The appropriate approach is to select based on full clinical profile, titrate slowly, and reassess if tolerability becomes problematic.

Who should consider Mounjaro versus Ozempic?

Tirzepatide appears to produce greater mean weight loss at the population level, based on two separate phase 3 trials and two independent systematic reviews. That is a meaningful clinical observation. It does not guarantee that Mounjaro will produce better results for any specific patient.

Several factors matter more for individual treatment selection than population-level averages:

Diabetes status: which medication is licensed and, where applicable, reimbursed for your diagnosis
Cardiovascular history: semaglutide has established cardiovascular outcomes data from published randomised trials; tirzepatide cardiovascular evidence is still emerging
Prior response to or tolerance of either medicine, if you have taken one before
Your baseline BMI and the degree of weight loss that would produce meaningful improvements in your metabolic markers
Contraindications, including personal or family history of medullary thyroid carcinoma or pancreatitis
Practical considerations: medicine availability in Estonia or Finland, cost, and your clinic's experience managing both options

This is a medical decision, not a consumer product comparison. Trial data provides a framework. A physician consultation translates that framework into an individual plan based on your specific profile.

Physician consultation

Not sure whether Mounjaro or Ozempic is right for you?

At Élan Clinic, we review your full metabolic profile before recommending any GLP-1 based treatment. If tirzepatide is likely a better fit, we will say so. If semaglutide makes more clinical sense, we will explain why.

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Sources

Wilding JPH, Batterham RL, Calanna S, et al. "Once-Weekly Semaglutide in Adults with Overweight or Obesity." New England Journal of Medicine. 2021;384(11):989-1002. PMID 33567185. DOI: 10.1056/NEJMoa2032183.
Jastreboff AM, Aronne LJ, Ahmad NN, et al. "Tirzepatide Once Weekly for the Treatment of Obesity." New England Journal of Medicine. 2022;387(3):205-216. PMID 35658024. DOI: 10.1056/NEJMoa2206038.
Aronne LJ, Sattar N, Horn DB, et al. "Continued Treatment With Tirzepatide for Maintenance of Weight Reduction in Adults With Obesity: The SURMOUNT-4 Randomized Clinical Trial." JAMA. 2024;331(1):38-48. PMID 38078870.
Look M, Dunn JP, Kushner RF, et al. "Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study of adults with obesity or overweight." Diabetes, Obesity and Metabolism. 2025. PMID 39996356. DOI: 10.1111/dom.16275.
Singh A, Singh AK, Singh R, et al. "Comparative efficacy and tolerability of tirzepatide versus semaglutide in obesity: an indirect treatment comparison." Diabetes & Metabolic Syndrome. 2025. PMID 40086043. DOI: 10.1016/j.dsx.2025.103212.
Moiz A, Filion KB, Toutounchi H, et al. "Efficacy and Safety of Glucagon-Like Peptide-1 Receptor Agonists for Weight Loss Among Adults Without Diabetes." Annals of Internal Medicine. 2025. PMID 39761578. DOI: 10.7326/ANNALS-24-01590.