Medication Comparisons

Mounjaro vs Retatrutide

Compare Mounjaro and Retatrutide, including how they work, expected results, pricing expectations, and what may change when Retatrutide launches.

Whether you are weighing up Mounjaro as your weight-loss treatment now, or watching retatrutide's development with interest, this guide compares them — what we know about each, where the comparison is meaningful, and where it isn't. The honest framing first: this is a comparison between a licensed UK medicine with strong evidence (Mounjaro) and an investigational medicine with no UK licence (retatrutide) — both, notably, from the same manufacturer (Eli Lilly). LetsLoseWeight is an independent comparison site; we do not prescribe medication.

Important: Retatrutide is not licensed by the MHRA and is not available from any regulated UK pharmacy. The comparison below is informational, drawing on Mounjaro's full UK licence and clinical trial record alongside retatrutide's published phase 2 data. Anyone offering retatrutide for sale in the UK outside a registered clinical trial is doing so unlawfully.

At-a-glance comparison

Mounjaro (tirzepatide) Retatrutide
UK status Licensed by MHRA (2022 for diabetes, weight management licensed 2023) Not licensed
Available in UK Yes, from regulated pharmacies No (clinical trial only)
Pivotal trial weight loss ~20.9% at 72 weeks on 15mg (SURMOUNT-1) ~24.2% at 48 weeks on 12mg (phase 2)
Receptors targeted GIP, GLP-1 GIP, GLP-1, glucagon
Trial size for weight management Thousands across SURMOUNT programme ~338 in phase 2 obesity trial
Real-world data Growing post-marketing record None
Manufacturer Eli Lilly Eli Lilly
Typical UK private price £130–£300 per month No legitimate UK price
NHS access Yes (TA1026, no 2-year cap) None

What's known about Mounjaro

Mounjaro is Eli Lilly's dual GIP/GLP-1 receptor agonist (tirzepatide), licensed for type 2 diabetes (2022) and weight management (2023). The pivotal SURMOUNT-1 trial showed average weight loss of:

  • 15.0% at 5mg over 72 weeks
  • 19.5% at 10mg
  • 20.9% at 15mg

All in adults with obesity but without type 2 diabetes (Jastreboff et al., NEJM 2022).

In the head-to-head SURMOUNT-5 trial in 2025, Mounjaro showed greater average weight loss than Wegovy (Aronne et al., NEJM 2025).

For more, see our Mounjaro results guide and Mounjaro side effects guide.

What's known about retatrutide

Retatrutide is Eli Lilly's investigational triple agonist, currently in phase 3 (the TRIUMPH programme). It activates three hormone receptors: GIP, GLP-1, and glucagon. The first two are also targeted by Mounjaro; the addition of glucagon receptor activation is what makes retatrutide novel.

The published phase 2 obesity trial showed average weight loss of around 24.2% at 48 weeks on the 12mg dose (Jastreboff et al., NEJM 2023).

The intuitive interpretation is "Mounjaro plus a third receptor = a bit more weight loss". The published phase 2 data is consistent with that, but with important caveats:

  • Phase 2 trials are smaller (~338 participants) than phase 3 (typically thousands)
  • Phase 2 trials are shorter — 48 weeks vs SURMOUNT-1's 72 weeks
  • Phase 3 results sometimes differ from phase 2 in larger, more diverse populations
  • No head-to-head comparison with Mounjaro has been published

For more, see our retatrutide results guide and retatrutide side effects guide.

Why "Mounjaro plus glucagon" is a more meaningful comparison than "Wegovy vs retatrutide"

Because Mounjaro and retatrutide share two of the three receptor targets, the incremental value of retatrutide vs Mounjaro is essentially the contribution of the glucagon component. This is more directly informative than comparing retatrutide with single-receptor Wegovy.

The hypothesised glucagon contribution:

  • Increased resting energy expenditure — burning more calories at rest
  • Increased fat breakdown in liver tissue (hence parallel trials for fatty liver disease)
  • Possible cardiometabolic effects through different mechanisms than GLP-1/GIP alone

Whether this incremental contribution is worth the additional clinical complexity — including some phase 2 signals around heart rate and arrhythmias at higher doses — is what phase 3 trials are designed to characterise.

Why direct cross-trial comparison is tricky

Comparing 20.9% (Mounjaro 15mg, 72 weeks) with 24.2% (retatrutide 12mg, 48 weeks) seems to favour retatrutide, but several factors make this less clear:

Different durations

SURMOUNT-1 ran 24 weeks longer than the retatrutide phase 2 trial. Mounjaro continued to produce weight loss through the late stages; retatrutide may show further weight loss out to 72 weeks too, or may plateau earlier. We don't know.

Different trial populations

Smaller phase 2 trials are sometimes more homogeneous than larger phase 3 trials, which can flatter results. Phase 3 may show smaller average effects.

No direct head-to-head

The strongest claim "retatrutide produces more weight loss than Mounjaro" requires a head-to-head trial. None has been published. Cross-trial comparisons should always be treated cautiously.

Phase 2 vs phase 3

Many drug candidates show somewhat smaller average effects in phase 3 than in phase 2 — sometimes for trial-design reasons, sometimes because the medicine is genuinely more variable in real-world populations.

A more honest summary: retatrutide phase 2 data suggests a step up over Mounjaro on average weight loss, but the effect size will only be reliably known after phase 3 readouts.

Side-effect comparison

Both medicines share the GLP-1 class profile dominated by gastrointestinal symptoms — nausea, diarrhoea, constipation, indigestion. The phase 2 retatrutide trial reported:

  • Broadly similar GI side-effect rates to Mounjaro at comparable points in titration
  • Some signals at higher doses worth watching in phase 3 — increased heart rate and a small number of arrhythmia events
  • Standard mechanism-class warnings likely to apply (thyroid C-cell tumours, MEN 2, pancreatitis, gallbladder)

For Mounjaro's side-effect profile in detail, see our Mounjaro side effects guide. The retatrutide profile will be characterised more fully through phase 3.

What about waiting for retatrutide?

A common question: "should I wait for retatrutide rather than starting Mounjaro now?"

The honest answer is: probably not. Specifically:

  • There is no published date for retatrutide approval in the UK
  • The wait could be measured in years
  • Mounjaro produces real, clinically meaningful weight loss for the majority of patients
  • Mounjaro is the largest-effect licensed weight-management medicine in the UK currently
  • Untreated obesity continues to cause harm during the wait
  • You can switch to retatrutide later if and when it becomes available, restarting at its starter dose

The decision worth making is "should I start Mounjaro?", not "should I wait for retatrutide?"

Practical implications now

The practical answer to "Mounjaro or retatrutide?" right now is:

  • Mounjaro is available, evidence-rich, and a strong choice for many patients
  • Retatrutide is not available legitimately in the UK at this time
  • The question is a real future decision, not a current one

Choosing now between Mounjaro and "an unregulated retatrutide vial from a non-pharmacy seller" isn't really a clinical decision — it's choosing between safety and risk. The MHRA has warned consumers about unregulated weight-loss-medicine sources.

For the live decisions among available options, see our Mounjaro vs Wegovy UK guide and best weight loss injection UK decision framework.

Frequently asked questions

Will retatrutide really produce more weight loss than Mounjaro?
Possibly, based on phase 2 data and the mechanism. But cross-trial comparison is unreliable, and phase 3 trials will give a more meaningful answer. Even if confirmed, "more weight loss on average" doesn't mean "more weight loss for every individual" — some patients respond better to one medicine than another.

Are the side effects worse with retatrutide?
Unknown. Phase 2 data suggests broadly similar gastrointestinal side effects. Some signals around heart rate and arrhythmias at higher doses are flagged for phase 3 examination. The full safety picture won't be known until phase 3 completes and post-marketing surveillance accumulates.

Should I switch from Mounjaro to retatrutide when it's licensed?
That decision will be available when retatrutide is licensed. At that point the choice becomes a real clinical decision based on actual data, your individual response to Mounjaro, side effects, cost, and prescriber recommendation. Don't pre-commit now.

Why is retatrutide more potent if it's the same manufacturer?
The triple-receptor target is the proposed mechanism. Eli Lilly developed retatrutide as a "next-generation" candidate. Whether the lab-derived advantage translates fully to clinical practice is what trials are designed to test.

Can I get retatrutide in a UK trial?
Possibly. The TRIUMPH trial programme is recruiting in some countries. For UK trial sites, see the National Institute for Health and Care Research's Be Part of Research portal. Trial participation is the only legitimate UK access.

Next steps

Sources

This guide is for general information only. Retatrutide is not currently approved or available in the UK outside clinical trials.

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Mounjaro vs Retatrutide UK: Cross-Trial Comparison | LetsLoseWeight