Tirzepatide vs Semaglutide: What the Science Actually Says About Weight Loss

The conversation around weight loss injections in the UK has never been more active. Two names come up more than any others: tirzepatide and semaglutide. You may know them better as Mounjaro and Wegovy or Ozempic, their respective brand names. But understanding what these compounds actually are, how they differ scientifically, and what the clinical evidence shows about their comparative effectiveness is genuinely useful if you are trying to make an informed decision about your own health.

What Is Semaglutide?

Semaglutide is a GLP-1 receptor agonist, a compound that mimics the GLP-1 hormone produced naturally in the gut after eating. It plays a central role in blood sugar regulation by stimulating insulin release, suppressing glucagon, and slowing gastric emptying. Read our explainer on what semaglutide is and how it works.

At the weight management dose of 2.4 mg per week (as found in Wegovy), semaglutide produces meaningful suppression of appetite and a sustained reduction in calorie intake. Ozempic contains the same compound at a lower dose and is licensed specifically for type 2 diabetes management.

What Is Tirzepatide?

Tirzepatide, sold in the UK under the brand name Mounjaro, is a dual-action compound. It activates both GLP-1 receptors and GIP receptors simultaneously. The GIP hormone is involved in insulin secretion and fat metabolism, giving tirzepatide a broader metabolic effect than GLP-1-only medications.

For a deeper look at how Mounjaro's dual mechanism affects the body, read our blog on how Mounjaro affects metabolism and energy levels.

What Do the Clinical Trials Actually Show?

The SURMOUNT-1 trial, which studied tirzepatide for weight management over 72 weeks, found the following average weight loss results:

• 5 mg dose: approximately 15% of body weight
• 10 mg dose: approximately 19.5% of body weight
• 15 mg dose: approximately 20.9% of body weight

The STEP-1 trial for semaglutide 2.4 mg over 68 weeks showed an average weight loss of approximately 14.9% of body weight.

On the basis of these trials, tirzepatide produced greater average weight loss than semaglutide at its highest dose. For a direct comparison from a clinical perspective, see our blog on whether Mounjaro is superior to Ozempic for weight loss.

The Caveats You Should Know

There is currently no large-scale, randomised, head-to-head trial comparing tirzepatide directly against semaglutide 2.4 mg for weight management in the same population. A network meta-analysis in The Lancet (2023) did compare the two indirectly and concluded that tirzepatide produced greater weight loss — but individual responses vary considerably.

For a broader look at the evidence, read our blog on Wegovy and Mounjaro: a closer look at clinical studies.

Side Effects: How Do They Compare?

The most commonly reported side effects for both medications are gastrointestinal: nausea, vomiting, diarrhoea, and constipation, particularly in the early weeks. Both use gradual dose escalation to reduce severity. Neither should be used by people with a personal or family history of medullary thyroid carcinoma or MEN2.

Which Is Available in the UK and How Do You Access Them?

Both Mounjaro (tirzepatide) and Wegovy (semaglutide 2.4 mg) are licensed in the UK for chronic weight management and available through the Pharmacy Planet online prescribing service. Both are prescription-only and require appropriate clinical assessment before use.

You can also explore weight loss options including Mysimba and Orlistat through Pharmacy Planet's weight loss range.

So Which One Should You Choose?

The clinical evidence suggests tirzepatide produces greater average weight loss. For people who have tried a GLP-1 medication without achieving adequate results, tirzepatide's dual mechanism may offer a meaningful step forward. However, semaglutide has a longer real-world track record and is highly effective in its own right. The right medication depends on your individual health profile, which your prescriber is best placed to assess.

Start a consultation with the Pharmacy Planet prescribing team at pharmacyplanet.com/start-consultation.html.

Frequently Asked Questions

Is tirzepatide stronger than semaglutide?

Clinical trial data suggests tirzepatide produces greater average weight loss than semaglutide 2.4 mg, particularly at higher doses. However, individual responses vary significantly.

Are Mounjaro and Wegovy the same thing?

No. Mounjaro contains tirzepatide (a dual GIP/GLP-1 agonist) and Wegovy contains semaglutide (a GLP-1 agonist only). They work differently and have different clinical profiles.

Can I switch from Wegovy to Mounjaro?

Switching is possible but must be done under prescriber guidance. Your prescriber will advise on timing, dose selection, and monitoring.

Are these medications available on the NHS?

Both are available in limited NHS settings through specialist weight management services. Most patients currently access them privately through services such as Pharmacy Planet.

How long do I need to take weight loss medication?

Both are intended for long-term use. Stopping treatment is associated with weight regain in most people. Read more about Mounjaro for long-term weight maintenance.

Reviewed for clinical accuracy. Content is for informational purposes only and does not constitute medical advice. All prescription medicines require a valid prescription following appropriate clinical assessment.