More

Best Weight Loss Injection UK

Compare the best weight loss injections in the UK, including Mounjaro and Wegovy, and find out which may be right for you.

Whether you are weighing up your first weight-loss injection or wondering whether the option you are already on is the right one, this guide is a decision framework for "best" — not a marketing claim. There is no single best weight-loss injection in the UK because "best" depends on what you weight most: average weight loss, side-effect tolerance, cost, eligibility, or how the medicine fits the rest of your life. LetsLoseWeight is an independent comparison site; we do not prescribe medication, and the right choice for you is a clinical decision with your prescriber.

The honest answer up front

If you optimise purely for average weight loss in published trials, the order is:

  1. Mounjaro (tirzepatide) — around 21% average weight loss at 15mg over 72 weeks (SURMOUNT-1)
  2. Wegovy (semaglutide) — around 14.9% average weight loss at 2.4mg over 68 weeks (STEP 1)
  3. Retatrutide — phase 2 only, ~24% at 12mg over 48 weeks, but not licensed in the UK

If you optimise for other things — tolerability, cost, NHS access, evidence base, longer track record — the answer changes. The rest of this page works through the criteria that actually matter for individual patients.

"Best" by average weight loss

The 2025 SURMOUNT-5 trial was the first head-to-head comparison of tirzepatide and semaglutide for weight management. After 72 weeks, average weight loss was greater with tirzepatide than with semaglutide, with both producing clinically meaningful weight loss (Aronne et al., NEJM 2025).

Mounjaro at 15mg therefore has the strongest published evidence for the largest average weight loss of any currently licensed UK weight-management medicine. But "average" hides a wide spread — some patients lose more on Wegovy than on Mounjaro, and a meaningful minority of patients are non-responders to either. For more, see our Mounjaro vs Wegovy UK guide.

"Best" by side-effect profile

The two medicines have broadly similar side-effect profiles, dominated by gastrointestinal symptoms. Specific differences:

  • Headache is very common with Wegovy and common with Mounjaro
  • Gallstones are common with Wegovy and uncommon with Mounjaro
  • Skin hypersensitivity reactions are slightly more frequent with Mounjaro

In SURMOUNT-5, gastrointestinal side effects were broadly comparable between the two, with some differences in severity at the highest doses. For most patients, tolerability differences are smaller than they expect. For dedicated side-effect detail, see our Mounjaro side effects and Wegovy side effects guides.

If you have type 2 diabetes, both medicines are appropriate but the choice may shift based on your existing medication regimen and your diabetes team's preference. If you have a history of gallbladder problems, Mounjaro may be slightly preferred. If you are sensitive to headache specifically, Mounjaro has a slightly lower frequency.

"Best" by cost

Private pricing for both medicines is broadly similar at equivalent doses, with provider-by-provider variation. As a rough range:

  • Mounjaro — typically £130–£300 per month depending on dose and provider
  • Wegovy — typically £130–£300 per month depending on dose and provider

For a multi-month course on the maintenance dose, total annual cost is similar between the two: roughly £2,500–£3,000 per year at typical pricing.

For NHS-eligible patients, the cost is dramatically different: only the standard prescription charge (£9.90 in 2025/26 in England; free in Scotland, Wales and Northern Ireland) for either medicine. NHS criteria differ slightly between Wegovy (TA875, capped at 2 years, accessed through specialist services) and Mounjaro (TA1026, accessed in primary care, no 2-year cap). For details, see our private vs NHS guide.

"Best" by eligibility

The licensed indications differ slightly between the two medicines:

  • Wegovy — adults with BMI ≥30, or ≥27 with at least one weight-related comorbidity (e.g. type 2 diabetes, hypertension, dyslipidaemia, obstructive sleep apnoea)
  • Mounjaro — adults with BMI ≥30, or ≥27 with at least one weight-related comorbidity

In practice the licensed eligibility criteria are very similar. NHS criteria are tighter — typically BMI ≥35 plus comorbidity for Wegovy, or BMI ≥35 with comorbidity (40 without) for Mounjaro under TA1026. See our eligibility guide for the full picture.

"Best" by track record

If you weight a longer track record:

  • Semaglutide (the active ingredient in Wegovy) has been in widespread use since 2017 (originally for type 2 diabetes as Ozempic). Long-term cardiovascular outcomes are well-characterised, including the SELECT trial showing a 20% reduction in major cardiovascular events (Lincoff et al., NEJM 2023).
  • Tirzepatide (Mounjaro) was approved more recently. Long-term cardiovascular outcomes are still being characterised through the SURMOUNT-MMO trial.

Both medicines are well-evidenced for weight management. Wegovy has more years of post-marketing surveillance behind it; Mounjaro has the larger weight-loss numbers. Both are reasonable choices.

"Best" by convenience and lifestyle fit

Both medicines are once-weekly subcutaneous injections, with very similar practical requirements:

  • Same injection sites — abdomen, thigh, upper arm
  • Similar pen design — pre-filled, single-use
  • Similar storage requirements — refrigerated, with a defined out-of-fridge window (21 days for Mounjaro, 28 days for Wegovy)
  • Similar dose-titration timelines — Wegovy reaches its maintenance dose at week 17, Mounjaro at week 17–20+ depending on the maintenance dose chosen

Practical differences are small. The licensed contraception warning is a Mounjaro-specific consideration: the SmPC recommends using a non-oral contraceptive method, or adding a barrier method, for 4 weeks after starting Mounjaro and 4 weeks after each dose increase if you take a combined oral contraceptive pill. There is no equivalent warning for Wegovy.

Why "best" doesn't include retatrutide

Retatrutide is not licensed in the UK. It is not available from any regulated UK pharmacy. The phase 2 trial figure of around 24% average weight loss is genuinely impressive, but it comes from a 48-week trial with around 80 participants per dose group — that is real evidence, but it is not the same as the multi-thousand-patient datasets behind Mounjaro and Wegovy, and phase 3 trials sometimes show different results.

Anyone offering retatrutide for sale in the UK is operating outside the regulated supply chain. The "best" weight-loss injection cannot reasonably be one you cannot legally and safely access. See our buy retatrutide UK guide for the full picture.

How to choose for yourself

If you are starting from scratch, a reasonable framework:

  1. Check eligibility for both with a regulated UK provider — see our eligibility guide.
  2. Consider NHS access first if you are eligible — the cost difference is substantial.
  3. Default to Mounjaro if you are paying privately, all else equal — the average weight-loss data favours it.
  4. Consider Wegovy if you have specific reasons: a history of skin hypersensitivity, a stable existing diabetes regimen built around semaglutide, a strong preference for the longer track record, or a meaningful price advantage at your chosen provider.
  5. Be prepared to switch if the medicine you choose first does not work for you — switching between the two is straightforward (see our switching guide).

What to ignore when choosing

Several "decision factors" that come up online are not actually useful:

  • "Mounjaro is newer therefore better" — newer doesn't always mean better. Both medicines have substantial published evidence behind them.
  • "Wegovy is the original therefore safer" — semaglutide has been in use longer in diabetes, but the safety profile of both medicines for weight management is well-characterised.
  • "My friend lost more on X" — individual response varies widely, and one or two examples don't predict your own response.
  • "Influencer X uses Y" — celebrities and influencers often have access to private prescribers and bespoke regimens that don't generalise. Their experience is not a reliable basis for your decision.

Frequently asked questions

Is Mounjaro definitely better than Wegovy?
On average for weight loss in head-to-head trials, yes — but individual response varies, side-effect profiles differ slightly, and "better" depends on what you optimise for. Both are reasonable choices.

Should I wait for retatrutide?
Generally, no. There is no published date for retatrutide approval in the UK, the wait is likely measured in years, and the medicines that exist now are well-evidenced and effective. Starting Mounjaro or Wegovy now and switching later if needed is a stronger plan than waiting for an unknown future option.

What if I can't tolerate Mounjaro — should I switch to Wegovy?
Possibly. Some patients tolerate one of the two medicines better than the other. Switching is straightforward and is sometimes successful. See our switching guide.

Is the cheapest option the best?
Cheapest is rarely the best in any meaningful sense. Within regulated UK pharmacies, the price difference is typically small, and the difference in aftercare and service can outweigh it. Cheapest unregulated is the worst possible choice — counterfeit risk, no clinical safety net, and potentially dangerous.

Next steps

Sources

This guide is for general information only and is not a substitute for professional medical advice. The right medicine for you is a clinical decision with your prescriber.

Get weekly weight loss tips

Join our newsletter for provider updates, expert guidance, and exclusive offers — no spam.

Subscribe free
Back to Prices
Best Weight Loss Injection UK: How to Decide | LetsLoseWeight