How tirzepatide and semaglutide differ

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. The trial results for each medicine are summarised below.

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 later trial, SURMOUNT-5, compared the two directly in 751 adults with obesity who did not have type 2 diabetes, each taking the highest dose they could tolerate for 72 weeks: average weight change was −20.2% with tirzepatide and −13.7% with semaglutide (Aronne et al., New England Journal of Medicine 2025). These are averages — individual responses vary considerably, and a trial result is not a recommendation for any individual.

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).

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 the medicine is licensed as an adjunct to a reduced-calorie diet and increased physical activity (each medicine's official product information on the electronic medicines compendium), so it is used alongside lifestyle changes, not instead of them.

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 how it compares with the licensed threshold. A consultation with our pharmacist is still required — this is a guide, not a clinical assessment.

What a trial average means

The estimator below shows the average weight loss reported with the highest tirzepatide dose in the SURMOUNT-1 trial, as a share of body weight.

It is an average from a controlled trial, run alongside diet and activity support — individual results vary, and some people lose more, some less. It is not a prediction of your own result.

How the choice is made

Trial averages do not decide which medicine, if any, suits a particular person. A prescriber considers, among other things:

• Your medical history, including any heart disease or type 2 diabetes — the heart outcome trials above studied different groups of people (SELECT: heart disease without diabetes; SURPASS-CVOT: type 2 diabetes with heart disease).
• Your other medicines, and anything in the official product information that means a medicine is not suitable for you.
• How you tolerate side effects as the dose is stepped up.
• Each medicine's licensed use and what you and the prescriber agree you want from treatment.

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 an article can decide for you.

How it works at Hyde Park Pharmacy

Four simple steps — led from start to finish by our pharmacists. No GP referral needed.

1 Step 1

Complete a quick assessment

Answer a few questions about your health, weight, and goals online. It only takes a few minutes.

2 Step 2

Speak with our pharmacists

Our pharmacists review your answers and meet you on a short video call, or in person, to decide whether treatment is appropriate for you.

3 Step 3

Discreet delivery to your door

If treatment is suitable, your medicine is dispensed by our pharmacy and delivered in plain, unmarked packaging.

4 Step 4

Ongoing support & reviews

We check in throughout your journey, track your progress, and adjust your treatment as needed.

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

Frequently asked questions

Do I need to see my GP first?

No. A pharmacy with a prescribing pharmacist can assess you without a GP referral. A full clinical consultation, by video or in person, is required before anything is prescribed, and the pharmacist decides whether any medicine is appropriate for you.

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. Report a suspected side effect from any medicine through the MHRA Yellow Card scheme. Hyde Park Pharmacy is regulated by the General Pharmaceutical Council (premises 9011727). Superintendent Pharmacist: Shoyab Umarji, GPhC #2065619.