Every weight-loss injection claims to work. The honest question is how much, over what timeframe, and how that compares once you set the marketing aside. When you're comparing weight loss injection efficacy, the trial data tells a clearer story than the adverts do — and it isn't the same story for every medicine.
This guide lines up the average weight loss reported in each medicine's pivotal trials, explains why those percentages aren't perfectly comparable, and shows what each option costs per month from our tracker of 24 regulated UK providers, last checked 01 September 2026.
Key takeaway: Tirzepatide (Mounjaro) produced the largest average weight loss in trials — around 21–22% of body weight at the higher doses over 72 weeks in SURMOUNT-1, and it came out ahead of semaglutide in the direct head-to-head SURMOUNT-5 trial (roughly 20% vs 14%). Semaglutide (Wegovy) still averaged a substantial ~15% in STEP 1 and has the longer track record. Higher average efficacy isn't automatically the right choice for everyone — side effects, tolerability and cost all matter too.
Why trial percentages aren't a perfect scoreboard
Each trial enrolled different people, over different lengths of time, with different starting weights and health conditions. SURMOUNT-1 excluded people with type 2 diabetes; some semaglutide trials didn't. A 72-week trial and a 68-week trial aren't measuring identically either. Treat the percentages below as a strong signal of relative efficacy, not a precise, apples-to-apples ranking.
Weight loss injection efficacy compared: the trial data
| Mounjaro (tirzepatide) | Wegovy (semaglutide) | |
|---|---|---|
| Key trial | SURMOUNT-1 | STEP 1 |
| Trial length | 72 weeks | 68 weeks |
| Average loss, top dose | 22.5% at 15 mg | ~15% at 2.4 mg |
| Average loss, mid dose | 21.4% at 10 mg | — |
| Lowest dose tested in trial | 16.0% at 5 mg | — |
| Placebo comparison | 2.4% on placebo | Not directly comparable here |
| Reaching ≥10% loss | ~90% of people on 15 mg | Not reported in the same terms |
| Head-to-head result (SURMOUNT-5) | ~20% average loss | ~14% average loss |
| Cheapest starter month (2.5 mg equivalent) | £145.99 (Click2Pharmacy) | See our full price comparison |
Figures are from the medicines' own published trial data. SURMOUNT-1 and STEP 1 enrolled different populations, so treat the percentages as a guide rather than a strict head-to-head — SURMOUNT-5 is the only direct comparison of the two medicines in the same study.
What drives the efficacy gap
Mounjaro's active ingredient, tirzepatide, acts on two gut hormone receptors (GIP and GLP-1) rather than one. Wegovy's semaglutide targets GLP-1 alone. That dual action is the leading explanation for why tirzepatide has, on average, produced greater weight loss in trials — though individual response varies a great deal, and some people lose more on semaglutide than the averages suggest. For a broader look at what separates the medicines beyond the numbers, see Mounjaro, Wegovy and the rest: what's actually different?
Efficacy isn't the only variable
A medicine that produces a slightly higher average loss isn't automatically the best fit. Side effects — nausea, diarrhoea, constipation, vomiting and tiredness are common across the class — can affect how consistently someone stays on treatment, which in turn affects real-world results more than trial percentages do. Our side-effect comparison breaks this down honestly. Rare but serious risks, including pancreatitis, gallstones, dehydration and low blood sugar in people also taking insulin or a sulfonylurea, apply across GLP-1 and dual-hormone medicines alike; any side effect should be reported through the MHRA Yellow Card scheme.
Dosing schedule matters too. Mounjaro starts at 2.5 mg for four weeks as a starter dose — not a treatment dose — before stepping up in 2.5 mg increments to a maintenance dose of 5, 10 or 15 mg, decided with a prescriber. Reaching a treatment dose, not just starting the injection, is what the trial percentages actually reflect.
What efficacy costs, month to month
Higher efficacy at the top doses generally means a higher ongoing price, since maintenance doses cost more than starter doses. It's worth comparing the full monthly cost — not just the headline starter price — before committing to a course. Our homepage price table tracks the 2.5mg Mounjaro starter dose across all 24 providers we monitor; the cheapest is Oxford Online Pharmacy at £143.96/4 weeks and the priciest is Numan at £219/4 weeks, a spread of £75.04 for the identical pen.
The Weight Clinic, our recommended provider, prices Mounjaro at £160/4 weeks with a first month of £125 using code NEWME, and offers a refund if the prescriber decides the medicine isn't suitable for you — a useful safety net when you're weighing efficacy against cost rather than guessing.
Choosing based on efficacy alone is risky
It's tempting to simply pick the medicine with the highest average trial percentage. But eligibility, tolerability, and how well a schedule fits your life all affect whether treatment actually delivers those results for you. Our 5-question framework for choosing a weight-loss injection walks through the practical checks worth doing before you compare percentages at all. For a wider snapshot of the whole UK market, see Best weight-loss injections in the UK: 2026 comparison.
Eligibility and prescribing
Mounjaro is a prescription-only medicine, licensed by the MHRA for weight management and recommended by NICE (TA1026, December 2024). Privately, it's typically available to adults with a BMI of 30+, or 27+ with a weight-related condition, following an assessment by a GPhC-registered pharmacy or CQC-regulated clinic. On the NHS it's being rolled out in phases from 2025, prioritising higher BMI and more weight-related conditions first, so access still varies by area. Nothing in this article is medical advice — a prescriber will decide whether any of these medicines are suitable for you.
Ready to compare properly? The Weight Clinic offers £35 off your first order with code NEWME, a refund if the prescriber declines your request, and monthly video reviews to check the dose and plan are still right for you.
FAQ
Which weight loss injection is most effective?
Based on trial averages, tirzepatide (Mounjaro) produced the greatest weight loss — around 21–22% of body weight at higher doses over 72 weeks, and it also outperformed semaglutide in the direct SURMOUNT-5 comparison. But individual response varies, and other factors like side effects and cost also matter.
Is a higher percentage weight loss always better?
Not necessarily. A medicine you tolerate well and can stay on consistently often produces better real-world results than one with a higher trial average that you struggle to continue.
How long does it take to see results?
Trial results are measured over many months — 68 to 72 weeks in the studies referenced here — with doses increased gradually. Early weeks are mainly about tolerating the starter dose, not judging final efficacy.
Do these efficacy figures apply to everyone?
No. Trial populations, durations and inclusion criteria differ, so the percentages are averages across specific groups of trial participants, not a guarantee for any individual.
Are the side effects worse for the more effective medicine?
Common side effects — nausea, diarrhoea, constipation, vomiting, tiredness — appear across the class, including Mounjaro and Wegovy. Report any side effect through the MHRA Yellow Card scheme.
Where can I compare prices for these medicines?
See our homepage price table and our dedicated price comparison across 24 UK providers.