Mounjaro vs Ozempic: clinical differences, effectiveness and which one suits you

Introduction

The arrival of GLP-1 receptor agonists in the treatment of obesity has transformed weight management medicine over the past five years. Within this category, two medicines account for practically all the media and clinical attention: Ozempic (semaglutide) and Mounjaro (tirzepatide). They are not the same. And understanding how they differ is crucial to seeing why the clinical decision is anything but trivial.

This comparison looks in detail at the real differences between the two medicines: mechanisms of action, effectiveness data from the pivotal trials, side effect profiles, availability in Spain and the clinical criteria for choosing one over the other depending on the patient. The information has been reviewed by Dr David Céspedes, Medical Director at AiON.

Background: the GLP-1 agonist 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 dosing frequency and the development of more sophisticated molecules.

Broadly speaking, they have evolved across three generations:

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

Ozempic (semaglutide): the key facts

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

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

Mounjaro (tirzepatide): the key facts

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

• Active substance: tirzepatide, a dual agonist that acts simultaneously on two receptors: GLP-1 and GIP (glucose-dependent insulinotropic polypeptide).
• Mechanism of action: it combines the effects of the classic GLP-1 agonists with a second hormonal pathway (GIP) that works synergistically on metabolism and satiety.
• Administration: weekly subcutaneous injection using a pre-filled multi-dose or single-dose pen.
• Approved indication: type 2 diabetes that is not controlled.
• Big brother: Zepbound (tirzepatide at higher doses, authorised for obesity in some countries, still under review in the EU at the time of publication).
• EMA authorisation: 2022.

The key difference: single agonist vs dual agonist

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

Semaglutide (Ozempic): selective GLP-1 agonism

It acts on the GLP-1 receptor only. This 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

It acts simultaneously on two different hormone receptors: GLP-1 (the same as semaglutide) and GIP. GIP is another incretin hormone that contributes to the metabolic response to food. Combining both mechanisms produces synergistic effects that are more powerful than either one on its own.

This is the molecular basis for why tirzepatide shows greater clinical effectiveness than semaglutide in head-to-head trials.

Effectiveness compared: the clinical trial data

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

SURMOUNT-1 trial (tirzepatide)

A phase 3 trial in 2,539 patients with obesity and without diabetes. Duration: 72 weeks. The mean body weight loss results were:

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

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

STEP-1 trial (semaglutide)

A phase 3 trial in 1,961 patients who were overweight or living with obesity and without diabetes. Duration: 68 weeks. The mean body weight loss results were:

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

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

SURMOUNT-5 trial (head-to-head comparison)

This 2025 trial compared tirzepatide and semaglutide head to head. Tirzepatide showed significant superiority for weight loss, with approximately 6.5 percentage points more weight loss (20.2% versus 13.7%) in the only head-to-head trial, SURMOUNT-5 (Aronne LJ et al., New England Journal of Medicine 2025) on average.

Side effect profile

Both medicines share a similar side effect profile, dominated by gastrointestinal effects, particularly during the dose escalation phase.

Common side effects (both)

• Nausea and vomiting, above all during the first few weeks.
• Diarrhoea or constipation.
• Reflux and a feeling of fullness.
• Fatigue early on.
• Pain or discomfort at the injection site.

Uncommon but serious side effects (both)

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

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

Availability in Spain and cost

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

Ozempic in Spain

Available in pharmacies with a prescription. Funded by the Spanish public health system (Seguridad Social) for type 2 diabetes under specific criteria. On a private prescription, the cost ranges roughly between 120 and 200 euros per monthly pen depending on the dose and presentation.

Mounjaro in Spain

Available in pharmacies with a prescription. Currently not funded by the Spanish public health system. The cost on a private prescription is similar to or slightly higher than Ozempic, roughly between 180 and 280 euros a month depending on the dose.

How the choice is made: the clinical decision

Choosing between Mounjaro and Ozempic is not the patient's decision but a clinical assessment that depends on multiple factors. The endocrinologist weighs up:

• The patient's treatment goal (blood sugar control, weight loss, or both).
• The degree of obesity or overweight and any associated comorbidities.
• The full medical history, including specific contraindications.
• How well similar medicines have been tolerated in the past.
• Whether the medicine is available at that particular time (there have been intermittent supply problems with both).
• Cost and coverage.
• The informed patient's preferences.

Broadly speaking, the available evidence suggests that tirzepatide offers greater average effectiveness for weight loss, but it is not the right choice for every profile. In some cases (patients who have previously had digestive intolerance, patients with specific medical histories, patients who meet particular funding criteria), semaglutide may be preferable.

Conclusion

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

That difference in effectiveness does not, however, make one medicine universally better than the other. The clinical choice depends on individual factors that only a specialist endocrinologist can properly assess, reviewing your full history, your goals, your contraindications and your likely response to each option.

Choosing the right treatment is a medical decision

At Peso, our medical team, supervised by Medical Director Dr David Céspedes, assesses your individual case and, if you are a clinical candidate for drug treatment, designs a personalised plan with the option best suited to your profile. Assessment in under 24 hours. If you are not a candidate, you pay nothing.

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Frequently asked questions

Is Mounjaro better than Ozempic for losing weight?

Data from head-to-head trials suggest that tirzepatide (Mounjaro) produces greater average weight loss than semaglutide (Ozempic). However, the right clinical choice depends on the individual patient's profile, contraindications, goals and tolerability. There is no universally better medicine; there is a medicine that is more suitable for each specific case.

Can you switch from Ozempic to Mounjaro?

Yes, under medical supervision. Switching between the two medicines follows a specific transition protocol that keeps side effects to a minimum. It must be directed by an endocrinologist and should never be done on your own.

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 its greater effectiveness for weight loss.

Which has fewer side effects, Ozempic or Mounjaro?

The side effect profile is similar for both, dominated by gastrointestinal effects in the early phase. Some data suggest a slightly higher incidence of gastrointestinal effects with Mounjaro at high doses, but the overall difference is modest when dose escalation is done correctly.

Which is more expensive in Spain?

Both are similarly priced on a private prescription (between 120 and 280 euros a month depending on the dose and presentation). Ozempic is funded by the Spanish public health system for type 2 diabetes under specific criteria. Mounjaro is not currently funded.

Your next step

The choice between one and the other is never the patient's: it is a clinical decision that depends on your case, your history and your goals. Medical weight loss treatment is now available in Spain, with a prior assessment, a prescription where appropriate from a registered doctor and ongoing follow-up from the medical team. You can check whether you are eligible in 30 seconds.

Legal notice

This content is for information and general education only. It is in no way a substitute for professional medical advice, diagnosis or treatment. The medicines mentioned in this article are prescription-only medicines and can only be dispensed with an official prescription issued by an authorised healthcare professional. Always consult an endocrinologist before starting any weight loss treatment.