Mounjaro vs Ozempic: clinical differences, efficacy and which is more appropriate

Reading time: 12 minutes. Last updated: April 2026. Reviewed by Dr. David Céspedes, Specialist in Preventive Medicine and Longevity.

Introduction

The arrival of GLP-1 receptor agonists in the treatment of obesity has transformed bariatric medicine in the last five years. Within this category, two medicines concentrate practically all the media and clinical attention: Ozempic (semaglutide) and Mounjaro (tirzepatide). They are different. And knowing how they differ is crucial to understanding why the clinical decision is not trivial.

This comparison analyses in detail the real differences between both medicines: mechanisms of action, efficacy data from pivotal trials, side-effect profiles, availability in Spain and clinical criteria for choosing one or the other depending on the patient. The information is reviewed by Dr. David Céspedes, a licensed endocrinologist specialised in preventive medicine and longevity.

Context: the GLP-1 agonists revolution

GLP-1 receptor agonists are not a recent discovery. The first molecule in this family (exenatide) was authorised in 2005 for type 2 diabetes. What has changed radically is the potency, the administration frequency and the development of more sophisticated molecules.

The evolution has followed approximately three generations:

• First generation: exenatide, liraglutide (Saxenda, Victoza). Daily administration. Moderate efficacy in weight loss (5 to 8%).
• Second generation: semaglutide (Ozempic, Wegovy). Weekly administration. Significant efficacy in weight loss (10 to 17% according to dose).
• Third generation (dual agonists): tirzepatide (Mounjaro, Zepbound). Weekly administration. Dual GLP-1 and GIP mechanism. Very high efficacy in weight loss (15 to 22%).

Ozempic (semaglutide): comparative factsheet

Ozempic is the trade name of semaglutide marketed by Novo Nordisk. It belongs to the second generation of GLP-1 agonists. Its key features:

• Active ingredient: semaglutide, a selective agonist of the GLP-1 receptor.
• Mechanism of action: mimics the body's GLP-1 hormone, stimulating satiety, slowing gastric emptying and acting on brain appetite centres.
• Administration: weekly subcutaneous injection, via a pre-filled pen.
• Official indication: type 2 diabetes not controlled with other treatments.
• Big brother: Wegovy (semaglutide at higher doses, authorised for obesity).
• EMA authorisation: 2018.

Mounjaro (tirzepatide): comparative factsheet

Mounjaro is the trade name of tirzepatide marketed by Eli Lilly. It belongs to the third generation, that of dual agonists. Its key features:

• Active ingredient: tirzepatide, a dual agonist that acts simultaneously on two receptors: GLP-1 and GIP (glucose-dependent insulinotropic peptide).
• Mechanism of action: combines the effects of classic GLP-1 agonists with a second hormonal pathway (GIP) that acts synergistically on metabolism and satiety.
• Administration: weekly subcutaneous injection, via multi-dose or single-dose pre-filled pen.
• Official indication: uncontrolled type 2 diabetes.
• Big brother: Zepbound (tirzepatide at higher doses, authorised for obesity in some countries, still in process in the EU at the date of publication).
• EMA authorisation: 2022.

The key difference: mono-agonist vs dual agonist

The most important technical distinction between both medicines is their receptor mechanism.

Semaglutide (Ozempic): selective GLP-1 agonism

Acts only on the GLP-1 receptor. It is what researchers call a mono-specific agonist. Its mechanism is limited to mimicking the GLP-1 hormone.

Tirzepatide (Mounjaro): dual GLP-1 and GIP agonism

Acts simultaneously on two different hormonal receptors: GLP-1 (the same as semaglutide) and GIP. GIP is another incretin hormone that contributes to the metabolic response to foods. The combination of both mechanisms produces more potent synergistic effects than either of them separately.

This is the molecular basis for which tirzepatide shows greater clinical efficacy than semaglutide in direct comparative trials.

Comparative efficacy: clinical trial data

The direct comparison between both medicines is backed by large-scale clinical trials. The two pivotal programmes are SURMOUNT (tirzepatide) and STEP (semaglutide).

SURMOUNT-1 trial (tirzepatide)

Phase 3 trial with 2,539 patients with obesity without diabetes. Duration: 72 weeks. The results of mean body weight loss were:

• Tirzepatide 5 mg: 15.0% loss.
• Tirzepatide 10 mg: 19.5% loss.
• Tirzepatide 15 mg: 20.9% loss.
• Placebo: 3.1% loss.

Source: Jastreboff et al., New England Journal of Medicine, 2022.

STEP-1 trial (semaglutide)

Phase 3 trial with 1,961 patients with overweight or obesity without diabetes. Duration: 68 weeks. The results of mean body weight loss were:

• Semaglutide 2.4 mg (Wegovy dose): 14.9% loss.
• Placebo: 2.4% loss.

Source: Wilding et al., New England Journal of Medicine, 2021.

SURMOUNT-5 trial (direct comparison)

This 2025 trial compared tirzepatide and semaglutide head-to-head. Tirzepatide showed significant superiority in weight loss, with approximately 47% more percentage loss on average.

Side-effect profile

Both medicines share a similar side-effect profile, dominated by gastrointestinal effects especially during the titration phase.

Common side effects (both)

• Nausea and vomiting, especially during the first weeks.
• Diarrhoea or constipation.
• Reflux and feeling of fullness.
• Initial fatigue.
• Pain or discomfort at the injection site.

Rare serious side effects (both)

• Acute pancreatitis.
• Gallbladder problems (cholelithiasis).
• Thyroid alterations (risk of medullary thyroid carcinoma in animal models).
• Allergic reactions.

The tolerability profile is similar between both. Tirzepatide may be associated with higher incidence of gastrointestinal effects at high doses, but the overall clinical difference is modest when titration is done properly.

Availability in Spain and cost

Both Ozempic and Mounjaro are authorised and available in Spain, but with different funding conditions.

Ozempic in Spain

Available at pharmacy with prescription. Funded by Social Security for type 2 diabetes with specific criteria. With a private prescription, the cost ranges approximately between €120 and €200 per monthly pen depending on dose and presentation.

Mounjaro in Spain

Available at pharmacy with prescription. Currently not funded by Social Security. The cost with a private prescription is similar or slightly higher than Ozempic, approximately between €180 and €280 per month depending on dose.

How to choose between them: the clinical decision

The decision between Mounjaro and Ozempic is not a patient choice, but a clinical assessment that depends on multiple factors. The endocrinologist considers:

• Therapeutic objective of the patient (glycaemic control, weight loss, both).
• Degree of obesity or overweight and associated comorbidities.
• Complete medical history, including specific contraindications.
• Previous tolerability to similar medicines.
• Availability of the medicine at the specific moment (there have been intermittent supply problems of both).
• Cost and coverage.
• Preferences of the informed patient.

In general, the available evidence suggests that tirzepatide offers greater average efficacy in weight loss, but it is not the right choice for all profiles. In some cases (patients with prior digestive intolerance, patients with specific history, patients with specific funding criteria), semaglutide may be preferable.

Conclusion

Mounjaro and Ozempic are two medicines from the same therapeutic family but with different mechanisms and differentiated clinical efficacy. Tirzepatide, by acting on two receptors (GLP-1 and GIP), shows on average greater efficacy in weight loss than semaglutide, according to direct comparative trials.

However, this difference in efficacy does not make one medicine universally better than the other. The clinical choice depends on individual factors that only a specialised endocrinologist can adequately assess, by reviewing your complete history, your objectives, your contraindications and your potential response to each option.

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Frequently asked questions

Is Mounjaro better than Ozempic for losing weight?

Direct comparative trial data suggest that tirzepatide (Mounjaro) produces greater average weight loss than semaglutide (Ozempic). However, the right clinical choice depends on the patient's individual profile, contraindications, objectives and tolerability. There is no universally better medicine; there is a more appropriate medicine for each specific case.

Can you switch from Ozempic to Mounjaro?

Yes, under medical supervision. The switch between both medicines is done with a specific transition protocol that minimises side effects. It is essential that it is directed by an endocrinologist and must never be done independently.

What does Mounjaro have that Ozempic does not?

The dual mechanism of action. Mounjaro acts simultaneously on the GLP-1 receptor (the same as Ozempic) and on the GIP receptor, a second incretin hormone. This dual action produces synergistic effects on satiety and metabolism that explain the greater efficacy in weight loss.

Which has fewer side effects, Ozempic or Mounjaro?

The side-effect profile is similar between both, dominated by gastrointestinal effects in the initial phase. Some data suggest slightly higher incidence of gastrointestinal effects with Mounjaro at high doses, but the overall difference is modest when titration is done correctly.

Which is more expensive in Spain?

Both have similar prices with a private prescription (between €120 and €280 per month depending on dose and presentation). Ozempic is funded by Social Security for type 2 diabetes with specific criteria. Mounjaro is currently not funded.

Legal notice

This content is informative and educational. It does not replace professional medical advice, diagnosis or treatment under any circumstances. The medicines mentioned in this article are drugs subject to medical prescription whose dispensation requires an official prescription issued by an authorised healthcare professional. Always consult an endocrinologist before starting any treatment for weight loss.