If your goal is a GLP-1 for maximum weight loss, the honest answer starts with the trial data: tirzepatide (Mounjaro) at its top dose has produced the largest average weight loss of any licensed GLP-1 in the UK. But "maximum" isn't just about picking the strongest molecule — it's about reaching and sustaining the right dose, sticking with treatment, and choosing a provider you can afford long term. This guide walks you through it, if-this-then-that style.
For the biggest average weight loss, Mounjaro (tirzepatide) at 15 mg leads the evidence — around 21–22% of body weight over 72 weeks in trials, versus around 15% for Wegovy (semaglutide 2.4 mg). A head-to-head trial (SURMOUNT-5) put tirzepatide at ~20% vs ~14% for semaglutide. The catch: results depend on titrating up slowly and staying on treatment. A prescriber decides what's suitable for you. This is information, not medical advice.
GLP-1 for maximum weight loss: what the trials actually show
Two medicines dominate the UK conversation, and the numbers are genuinely different.
- Tirzepatide (Mounjaro) — In SURMOUNT-1, adults with obesity or overweight (without type 2 diabetes) lost on average 16.0% at 5 mg, 21.4% at 10 mg and 22.5% at 15 mg over 72 weeks, versus 2.4% on placebo. About 90% of people on 15 mg lost at least 10% of their body weight.
- Semaglutide (Wegovy) — In STEP 1, the 2.4 mg dose produced around 15% average weight loss.
- Head-to-head — SURMOUNT-5 compared the two directly and found roughly 20% vs 14% in favour of tirzepatide.
So if the single question is "which molecule delivers the most on average?", tirzepatide at a high maintenance dose is the front-runner. Tirzepatide works on two gut hormone pathways (GIP and GLP-1), while semaglutide targets one — a plausible reason for the gap.
Trial figures are averages. Some people lose far more, some less, and nobody is guaranteed a specific result. "Maximum weight loss" depends heavily on reaching a treatment dose, tolerating it, and combining the medicine with diet and activity changes. Report any side effects through the MHRA Yellow Card scheme.
Dose matters more than most people realise
The eye-catching 22.5% figure comes from the 15 mg dose — not the starter. Mounjaro is deliberately titrated slowly:
- 2.5 mg weekly for 4 weeks — a starter dose to settle your stomach, not a treatment dose.
- 5 mg next, then steps of 2.5 mg (7.5, 10, 12.5) after at least 4 weeks each.
- Up to 15 mg maximum. Your maintenance dose (5, 10 or 15 mg) is tailored to you.
The practical takeaway: chasing maximum results means budgeting for the higher doses, because that's where the biggest numbers live — and higher doses cost more per month. Rushing the titration usually backfires, since nausea, diarrhoea and vomiting are most likely when you step up too fast.
If you can't tolerate the top dose
Plenty of people do very well on 10 mg and never need 15 mg. If side effects are a problem, a prescriber may hold you at a lower dose for longer or, occasionally, consider switching — our guide to switching between GLP-1 medications explains how that works. The "best" dose is the highest one you can comfortably sustain.
Which medicine should you actually choose?
Use this quick decision tree:
- You want the largest average weight loss and can tolerate injections? → Mounjaro is the evidence leader.
- You've tolerated semaglutide well before, or your prescriber recommends it? → Wegovy is still a strong, well-studied option.
- You're unsure where to start? → Work through five questions that decide your GLP-1 shortlist, or read which GLP-1 is right for me for an honest walk-through.
Eligibility is broadly a BMI of 30+, or 27+ with a weight-related condition. If you're in the lower band, see BMI 27–30: your GLP-1 options explained. Mounjaro is MHRA-licensed and recommended by NICE (TA1026); NHS access is rolling out in phases from 2025, so most people going privately are paying out of pocket.
Because you'll likely be on treatment for many months to reach and hold a high dose, the provider's price at the upper doses matters as much as the starter price. The Weight Clinic, our recommended provider, keeps its higher-dose Mounjaro pricing competitive, which matters when you're aiming for 15 mg.
What the top dose costs across UK providers
Since maximum weight loss lives at the higher doses, the table below shows real ongoing prices per 4 weeks for Mounjaro 15 mg from a sample of UK-regulated providers, alongside the 2.5 mg starter for reference. Prices were last checked 17 July 2026 — always confirm on the provider's own site. You can compare the full list on our homepage price table.
| Provider | 2.5 mg starter / 4 wks | 15 mg top dose / 4 wks | Notes |
|---|---|---|---|
| Medicine Marketplace | £137 | £266 | GPhC pharmacy |
| Next Script | £131 | £266 | Cheapest starter |
| Totiva Health | £136.88 | £268.88 | GPhC pharmacy |
| PillSphere | £135 | £268 | GPhC pharmacy |
| Get Weightloss | £133.99 | £269 | GPhC pharmacy |
| Fylde Clinic | £134 | £270 | GPhC pharmacy |
| The Weight Clinic | £160 (first month £125) | £295 | Recommended · code NEWME |
| Boots Online Doctor | £179 | £339 | GPhC + CQC |
| Numan | £219 | £359 | GPhC + CQC |
The spread is large: at 15 mg you'll find prices from the mid-£260s up to £359 for the identical pen. Cheapest isn't automatically best — factor in prescriber support, review appointments and whether the low headline price holds once you're at your treatment dose.
Aiming for maximum results? Start with proper support.
The Weight Clinic is our recommended provider for Mounjaro and Wegovy, with monthly video reviews so a prescriber can help you titrate up safely toward your maintenance dose. Use code NEWME for £35 off your first order, and if you're declined as unsuitable you're refunded. See The Weight Clinic.
Start a free consultation →Frequently asked questions
Which GLP-1 gives the most weight loss?
On average, tirzepatide (Mounjaro) at its 15 mg dose has produced the largest weight loss of the licensed UK options — around 21–22% over 72 weeks in trials, compared with about 15% for semaglutide (Wegovy). A head-to-head trial found roughly 20% vs 14% in tirzepatide's favour.
Do I have to reach 15 mg to lose weight?
No. Even the 5 mg dose produced around 16% average loss in trials, and many people do very well on 10 mg. The dose is titrated slowly and your best maintenance dose is the highest one you can comfortably tolerate — not necessarily the maximum.
Will I definitely lose 20% of my body weight?
No. Trial figures are averages — some people lose more, some less, and results are never guaranteed. Weight loss depends on reaching a treatment dose, tolerating it, and combining the medicine with diet and activity changes. A prescriber decides suitability.
How much does the top dose cost in the UK?
Mounjaro 15 mg ranges from the mid-£260s to around £359 per 4 weeks across UK-regulated providers. Because you'll be on treatment for months, it's worth checking a provider's higher-dose prices, not just the starter price.
What are the common side effects?
The most common are nausea, diarrhoea, constipation, vomiting and tiredness — usually worst when stepping up a dose. Rarer serious issues include pancreatitis and gallstones. Report any side effects via the MHRA Yellow Card scheme and speak to your prescriber if symptoms are severe.
Can I switch from Wegovy to Mounjaro for bigger results?
Some people do, but it's a decision for your prescriber, who will restart you at a low dose and titrate up again. Our guide to switching between GLP-1 medications explains the practicalities.