Tirzepatide vs semaglutide: which weight loss injection is more effective?

Two injections dominate the conversation about prescription weight management in the UK right now: tirzepatide, sold as Mounjaro, and semaglutide, sold as Wegovy. Both are once-weekly injections. Both require a prescription and a clinical assessment. And both produce meaningful weight loss in the right patients.

But they are not the same medicine, and the difference between them matters. This article explains exactly how they differ, what the evidence says about outcomes, who is eligible for each, and how to use our free BMI checker to see where you sit.

The fundamental difference: one receptor vs two

Both medicines mimic naturally occurring hormones involved in appetite and blood-sugar regulation — but they do so at different receptors.

Semaglutide (Wegovy) is a GLP-1 receptor agonist. GLP-1 (glucagon-like peptide-1) is a gut hormone released after eating. It tells the brain you're full, slows gastric emptying, and stimulates insulin release. By mimicking this hormone, semaglutide reduces hunger and calorie intake over time.

Tirzepatide (Mounjaro) is a GLP-1 and GIP dual agonist. It acts on both the GLP-1 receptor and a second receptor: GIP (glucose-dependent insulinotropic polypeptide). GIP also plays a role in appetite regulation, fat storage, and energy balance. Acting on both receptors together appears to produce a more powerful combined effect than either alone — which is reflected in the trial data.

What the clinical trials actually show

The weight-loss trials for both medicines are large, well-designed, randomised controlled trials lasting over a year. Here's what they found:

In concrete terms: at the highest licensed dose, tirzepatide produced an average weight loss of approximately 22.5% of body weight over 72 weeks. Semaglutide produced approximately 15% of body weight over 68 weeks. Both are substantially better than placebo (around 2–3%).

A direct head-to-head trial (SURMOUNT-5) subsequently confirmed tirzepatide's advantage: participants on tirzepatide lost around 47% more weight than those on semaglutide over the same period. These are averages — individual responses vary considerably.

Side-by-side comparison

Side effects: what to expect with both

Both medicines share a very similar side-effect profile. The most common are gastrointestinal: nausea, diarrhoea, constipation, and vomiting. These are most pronounced during the dose-escalation phase — when your dose is being stepped up over the first few weeks — and typically improve as your body adjusts.

Practical points:

• Eating smaller meals, avoiding fatty or spicy food, and staying hydrated all help manage GI symptoms.
• Both injections are administered in the thigh, abdomen, or upper arm — rotating sites helps reduce skin reactions.
• Rare but important risks include pancreatitis (stop immediately and seek urgent medical advice if you develop severe abdominal pain) and gallbladder problems (including gallstones).
• Both are contraindicated in personal or family history of medullary thyroid carcinoma or Multiple Endocrine Neoplasia type 2 (MEN2).
• Tirzepatide may cause slightly more nausea at higher doses; semaglutide may cause slightly more constipation — but the difference is modest.

Who qualifies? Eligibility for both treatments in the UK

The standard UK eligibility criteria for both Mounjaro and Wegovy are:

• BMI of 30 or above (obesity), or
• BMI of 27 or above with at least one weight-related comorbidity — such as type 2 diabetes, hypertension (high blood pressure), dyslipidaemia, obstructive sleep apnoea, or a history of cardiovascular disease

In both cases you must also have tried lifestyle interventions (diet and exercise) without sufficient effect, and be committed to continuing lifestyle changes alongside the medicine.

There are additional contraindications — including severe renal or hepatic impairment, a history of eating disorders, certain medication interactions, and others — which is why a full clinical consultation is required rather than just a BMI check.

Check your BMI

Use the calculator below to find your BMI and see whether you meet the standard eligibility threshold. A consultation with our pharmacist is still required — this is a guide, not a clinical assessment.

How much weight could you lose?

In the largest clinical trial of Mounjaro (called SURMOUNT-1), people taking the highest dose lost nearly a quarter of their body weight on average — over about 18 months. To put that in real terms: if you start at 90 kg, that is roughly 20 kg lost. If you start at 120 kg, it is closer to 27 kg.

These are averages from a controlled trial — individual results vary, and some people lose more, some less. But it gives you a realistic sense of the scale of change that is possible with the right treatment and support.

Use the slider below to enter your starting weight and see what 22.5% looks like for you.

Which one should you choose?

Based on the evidence alone, tirzepatide (Mounjaro) produces greater average weight loss. For most people without specific contraindications, that makes it the first-choice option when the goal is maximum weight reduction.

However, semaglutide (Wegovy) has several things in its favour:

• A longer safety track record — semaglutide has been in use for type 2 diabetes since 2017, giving us several more years of post-marketing data.
• Completed cardiovascular outcome data — the SELECT trial showed semaglutide reduces the risk of major cardiovascular events by around 20% in people with existing cardiovascular disease, an effect tirzepatide does not yet have published data for.
• Some people tolerate semaglutide's side-effect profile better than tirzepatide's — particularly at higher doses.

The right choice is an individual one, made in consultation with a pharmacist or doctor who can review your full medical history, current medications, and personal goals. It is not something a comparison article can decide for you.

What happens at our pharmacy

At Hyde Park Pharmacy our weight management service is led by our superintendent pharmacist Shoyab Umarji, who is an Independent Prescriber (GPhC #2065619). This means a GP referral is not required.

The process is:

1. You complete a clinical questionnaire online (health history, current medications, eligibility criteria).
2. You book a video consultation at a time that suits you — our pharmacist reviews your answers and speaks with you face-to-face over video.
3. If treatment is appropriate, a prescription is issued and the medicine is dispensed or dispatched to you.
4. Ongoing monitoring is arranged — both medicines require regular review during the dose-escalation phase.

We do not supply weight management medicines without a consultation. Lifestyle support is part of the package, not an afterthought.

Frequently asked questions

Is Mounjaro better than Wegovy?

In terms of average weight loss, yes — the trial data consistently shows tirzepatide (Mounjaro) produces greater weight reduction than semaglutide (Wegovy). However, "better" in medicine depends on the individual. Semaglutide has a longer safety record and completed cardiovascular outcome data. A pharmacist will help you weigh up which is more appropriate for your specific situation.

Can I get Mounjaro from a pharmacy without going to my GP?

Yes. As an Independent Prescriber pharmacy, Hyde Park Pharmacy can assess, prescribe and supply Mounjaro and Wegovy without a GP referral. A full clinical consultation — including a video call — is required before any prescription is issued.

How long do I need to take the injection?

Both medicines are intended as long-term treatments. Trials show that weight regain occurs when the medicine is stopped, in most patients. The current clinical view is that treatment should continue as long as benefit is maintained and there are no safety concerns — in practice, this often means years rather than months. This is one of the most important conversations to have at consultation.

Does it work without changing my diet?

Diet and exercise significantly amplify the weight loss from both medicines. The trials were conducted alongside lifestyle counselling — the headline weight loss figures are not from medication alone. That said, the appetite-suppressing effect of both medicines does make it meaningfully easier to eat less, which is why many people find the combination more achievable than diet alone had been.

This article is written and reviewed by our pharmacist team for general information purposes. It does not constitute medical advice and does not replace an individual clinical consultation. Always seek the advice of a qualified healthcare professional about your specific circumstances before starting any new medicine. Hyde Park Pharmacy is regulated by the General Pharmaceutical Council (premises 9011727). Superintendent Pharmacist: Shoyab Umarji, GPhC #2065619.