Mounjaro vs Wegovy: a real comparison based on trial data

What SURMOUNT-1 and STEP-1 actually showed

Both drugs were tested in large randomised placebo-controlled trials. The headline numbers are widely quoted but rarely placed side by side honestly.

SURMOUNT-1 (Mounjaro / tirzepatide, published New England Journal of Medicine 2022): 2,539 adults with BMI of 30 or above (or 27+ with a weight-related comorbidity), no diabetes. At 72 weeks: 15mg dose group lost an average of 20.9% of body weight; 10mg group lost 19.5%; 5mg group lost 15.0%. Placebo lost 3.1%. Notably, around 57% of patients on 15mg achieved 20% or more weight loss.

STEP-1 (Wegovy / semaglutide, NEJM 2021): 1,961 adults with BMI 30+ (or 27+ with comorbidity), no diabetes. At 68 weeks: 2.4mg dose group lost 14.9% of body weight; placebo lost 2.4%. Around 32% of patients achieved 20% or more weight loss.

So Mounjaro 15mg vs Wegovy 2.4mg, at near-identical durations: roughly 21% vs 15% body-weight loss. That's a real difference, but it's also a comparison of two trials run by different sponsors with different protocols — not a head-to-head. There's now one published head-to-head trial (SURMOUNT-5, presented 2025) which compared Mounjaro and Wegovy directly and found a similar gap, roughly 20% vs 14% at 72 weeks.

Mechanism: GLP-1 alone vs dual GLP-1/GIP

Wegovy (semaglutide) is a pure GLP-1 receptor agonist. It mimics the natural hormone glucagon-like peptide 1, slowing gastric emptying, increasing satiety, reducing appetite, and improving insulin response to food.

Mounjaro (tirzepatide) does all of that and also activates the GIP (glucose-dependent insulinotropic polypeptide) receptor. GIP is the other major incretin hormone. The clinical question scientists asked for years was whether adding GIP would add to GLP-1's effect or cancel it out (GIP can either help or worsen weight depending on context). Tirzepatide answered the question empirically: adding GIP agonism on top of GLP-1 agonism produces more weight loss, better glycaemic control, and a slightly different side-effect profile. The GIP component appears to improve insulin sensitivity, reduce inflammation in adipose tissue, and modulate fat storage in ways pure GLP-1 doesn't.

Side effect profile differences

Both drugs share the GLP-1 side-effect family — nausea, indigestion, constipation, diarrhoea, occasional sulphur burps. The intensity peaks during dose titration and settles for most patients within 4–8 weeks. In the trial data:

• Nausea: Mounjaro slightly higher in early titration (around 25% vs 20% in headline rates), but the absolute difference is small and depends on titration speed.
• Vomiting and diarrhoea: Similar between the two; both elevated vs placebo.
• Gallbladder problems (cholelithiasis, cholecystitis): Rare for both — around 0.4–0.6% in both trials, vs 0.1–0.2% on placebo. Rapid weight loss itself is a risk factor.
• Pancreatitis: Very rare. Boxed warning on both; clinical incidence well under 0.1% per patient-year.
• Thyroid C-cell tumours: Theoretical risk based on rodent studies; not demonstrated in human populations to date. Both are contraindicated in patients with personal or family history of medullary thyroid carcinoma or MEN-2.

For practical day-to-day management of nausea and indigestion, see our GLP-1 side effects timeline guide.

Dose escalation schedules

Mounjaro: Start at 2.5mg weekly for four weeks; increase to 5mg, then 7.5mg, 10mg, 12.5mg, 15mg in four-week steps. The titration is gentle by design — most patients stop somewhere between 7.5mg and 12.5mg as the effective dose. Going to full 15mg is for those who want the maximum effect and tolerate it well.

Wegovy: Start at 0.25mg weekly for four weeks; increase to 0.5mg, 1mg, 1.7mg, 2.4mg in four-week steps. So 16 weeks to reach the trial-level dose. Some patients hold at 1.7mg if they're losing well and tolerating it; others progress to 2.4mg.

Cost differential at full dose

At maximum trial dose, both drugs are expensive privately. Mounjaro's per-mg cost is lower than Wegovy's because the doses are higher numerically, but you also need more drug. In practice at full dose, the monthly cost is broadly similar but slightly higher for Mounjaro. We discuss specific monthly costs at consultation; see our pricing page for the structure. The bigger cost question is sustainability: both drugs require ongoing use to maintain weight loss (see what happens when you stop Mounjaro), so plan for the medium term.

NICE eligibility: TA1026 vs TA875

NICE TA875 (semaglutide / Wegovy): NHS eligible for adults with BMI 35+ with at least one weight-related comorbidity, or BMI 30–34.9 in specialist weight-management services. Provided through specialist NHS weight-management clinics for a maximum of two years.

NICE TA1026 (tirzepatide / Mounjaro): Similar to Wegovy but with phased rollout starting from 2024. NHS eligible for BMI 35+ with one comorbidity, accessed through specialist services. From mid-2025, the rollout has been progressively expanded to primary care in some regions.

Both eligibility frameworks use the standard BMI thresholds — and importantly, both apply the NICE CG189 ethnicity adjustment: BMI thresholds are reduced by 2.5 points for South Asian, Chinese, Middle Eastern, Black African and African-Caribbean adults. So BMI 27.5 with comorbidity replaces 30, and BMI 32.5 unconditional replaces 35. This matters enormously for our patients in Rusholme. See our full NICE CG189 BMI thresholds guide.

When Wegovy is the right call despite Mounjaro's edge

Mounjaro produces more weight loss on average. But average isn't everything. There are real situations where Wegovy is preferable:

• Longer real-world track record. Wegovy has been in widespread use since 2021; tirzepatide since 2022 in the US, mid-2024 in UK retail. If post-marketing surveillance reassurance matters to you, Wegovy has more years of it.
• Slower titration if you're side-effect sensitive. Wegovy's lower starting dose can feel gentler.
• Cardiovascular outcomes evidence. The SELECT trial (2023) showed Wegovy reduces major adverse cardiovascular events by around 20% in adults with overweight and established cardiovascular disease. Tirzepatide has SURMOUNT-MMO ongoing but the evidence base is less mature.
• If you've responded well already. Switching just to chase the extra few percent isn't usually justified mid-treatment.
• Supply. Both have had intermittent supply issues since launch; whichever is reliably available can win by default.

Baseline blood tests before starting either

We run a baseline panel before starting either drug — see our pre-Mounjaro baseline panel guide for what's included and why. The same panel applies to Wegovy. Book at Trafford Clinic blood tests in Manchester.

How to decide — and how we'd advise you

Our usual approach in consultation: if your goal is the maximum likely weight loss and you tolerate medication well, we lean towards Mounjaro. If you have an established cardiovascular condition and want the trial evidence behind your choice, we lean towards Wegovy. If cost matters, we discuss both honestly and let you decide. We have weight-loss clinics across Greater Manchester — Manchester city centre, Altrincham, Salford Quays, Sale, Chorlton, Stretford, Rusholme, Eccles, Whalley Range, Firswood, and Old Trafford.

Book a free initial consultation at traffordclinic.co.uk/weight-loss or call 0161 258 6149. Empire Pharmacy is GPhC-registered (premises 1123966); Haroon Iqbal is an Independent Prescriber (reg. 2051093).

Related reading: What happens when you stop Mounjaro, GLP-1 side effects timeline, Travelling with Mounjaro or Wegovy.