Retatrutide vs Mounjaro: How Do the Two Eli Lilly Weight Loss Jabs Actually Compare?

If you have been following the weight loss medicine story closely, you will have noticed that retatrutide and Mounjaro keep appearing in the same conversation. Both are made by Eli Lilly. Both act on GLP-1 receptors. And trial data for retatrutide suggests it may produce greater weight loss than Mounjaro. But the two medicines are not interchangeable, and for UK readers the practical differences matter enormously.

How the Two Medicines Work Differently

Mounjaro (tirzepatide) is a dual agonist. It activates two hormone receptors simultaneously: GLP-1, which reduces appetite and slows gastric emptying, and GIP, which amplifies the insulin response and appears to enhance weight loss beyond GLP-1 alone. This dual action is why Mounjaro tends to produce greater weight loss than older GLP-1-only medicines such as Ozempic or Wegovy.

Retatrutide goes a step further. It is a triple agonist, activating GLP-1, GIP and glucagon receptors. The addition of glucagon receptor activity is significant because glucagon increases energy expenditure in addition to affecting appetite. In theory, targeting all three pathways simultaneously could produce greater weight reduction than a dual agonist alone — though real-world results across diverse populations will ultimately determine how meaningful that difference is.

What the Trial Data Actually Shows

The most important clinical trial for Mounjaro is SURMOUNT-1, published in the New England Journal of Medicine in 2022. In that trial, participants taking the highest dose of tirzepatide (15mg weekly) achieved a mean weight loss of around 20.9% of body weight over 72 weeks. This was a landmark result — substantially better than anything seen in GLP-1-only trials at the time.

For retatrutide, the earlier Phase 2 trial, also published in the New England Journal of Medicine in 2023, found that participants taking the highest dose (12mg weekly) achieved a mean weight loss of approximately 24.2% of body weight at 48 weeks. Eli Lilly’s Phase 3 programme, known as TRIUMPH, has since reported. TRIUMPH-1, in people with obesity but without diabetes, found 28.3% average weight loss at 80 weeks on 12mg (topline results announced in May 2026; full results published in the New England Journal of Medicine in September 2026). TRIUMPH-2, in people with obesity and type 2 diabetes, was published in full in The Lancet in September 2026: 20.8% average weight loss on 12mg if people stayed on treatment, or 18.8% counting everyone who started the trial.

It is important to be clear about what these numbers do and do not tell us. These are separate trials with different participant populations, different dose escalation schedules, different trial durations and different endpoints. They were not designed to compare retatrutide and Mounjaro head to head. Presenting the Phase 2 retatrutide figure alongside the SURMOUNT-1 figure as though they are directly comparable would be misleading. The same caution applies to the Phase 3 TRIUMPH results. They give a much clearer picture than Phase 2 alone, but none of these trials compared retatrutide directly with Mounjaro, and so far only TRIUMPH-1 and TRIUMPH-2 have been published in full.

Side Effect Profiles: Similar but Not Identical

Both medicines share a recognisable side effect profile. Nausea, vomiting, diarrhoea and constipation are the most commonly reported effects, particularly during dose escalation. These are consistent with GLP-1 receptor agonism and are well documented across this class of medicine.

Retatrutide’s additional glucagon receptor activity adds a dimension that is less well characterised at scale. The Phase 2 trial did not raise unexpected safety signals, but Phase 2 trials are not powered to detect rarer effects. The Phase 3 trials have since added a much larger safety picture: digestive side effects such as nausea and diarrhoea were most common, and an altered skin sensation called dysesthesia was reported more often than with placebo. Rarer and longer-term effects in a broad population will only become clear with longer follow-up and post-marketing surveillance.

Mounjaro’s safety profile is now supported by a much larger body of evidence, including post-authorisation real-world data from markets where it has been available for longer. For a medicine you are thinking about taking now, that difference in the evidence base matters.

UK Availability: A Fundamental Difference

Mounjaro received MHRA authorisation in the UK for weight management in adults with obesity or overweight with at least one weight-related health condition. It is available today through licensed UK private clinics and, for eligible patients, through NHS specialist weight management services following NICE guidance. You can read more in our Mounjaro UK guide.

Retatrutide has not received regulatory approval from any medicines regulator in the world, including the MHRA, the FDA or the EMA. There is currently no MHRA-authorised retatrutide medicine available in the UK. Eli Lilly plans to apply to the US FDA in early 2027 but has not announced a UK application date. Until retatrutide receives a UK marketing authorisation, it cannot be legally prescribed or dispensed in the UK through normal authorised channels. You can read more about where things currently stand in our Retatrutide UK guide.

The Risks of Grey-Market Retatrutide

Because retatrutide has attracted significant interest, peptide products labelled as retatrutide are being sold online and through informal channels. These products are not MHRA-authorised medicines. Their identity, purity, strength and contents cannot be reliably verified without independent testing. There is no regulated supply chain, no licensed manufacturer producing them for human use in the UK, no clinical oversight and no legal framework protecting buyers.

Regulators and clinicians have raised serious concerns about unverified injectable peptide products more broadly. Contamination, incorrect dosing and misidentified compounds are all documented risks with products obtained outside authorised pharmaceutical supply chains. Anyone considering accessing retatrutide through informal online sources is taking on risks that are currently impossible to fully quantify — and doing so without clinical supervision or any means of verifying what they are actually injecting.

If You Are Currently on Mounjaro: What to Understand About Retatrutide

For people currently on Mounjaro who are aware of retatrutide and wondering whether to switch, the honest answer is that no switch is possible through legitimate UK channels right now. Retatrutide is not available as an authorised medicine. Even with Phase 3 results now reported, a regulatory submission, MHRA review, potential NICE appraisal and NHS commissioning process would follow before broad UK access becomes realistic. Each of those stages takes time.

If retatrutide eventually receives MHRA authorisation and NICE recommends it, a switch from Mounjaro to retatrutide would be a clinical decision made with your prescriber, based on your individual circumstances, how you have responded to Mounjaro, any side effects you have experienced and the evidence available at that time. It would not be an automatic or straightforward process, and it would require proper medical supervision given that both medicines affect appetite, weight and metabolic function in ways that need to be managed carefully during any transition.

The most useful thing someone on Mounjaro can do right now is focus on the treatment they are taking. Mounjaro has a strong and growing evidence base. If it is working well for you and your prescriber is satisfied, there is no clinical reason to be looking elsewhere.

What This Means For UK Patients

Retatrutide is a genuinely interesting medicine in development, and its Phase 3 results so far are strong. But trial results are not the same as real-world experience, and long-term safety data in a broad population does not yet exist. Cross-trial comparisons between retatrutide and Mounjaro — while often cited — are not head-to-head results and should not drive treatment decisions.

For UK patients today, Mounjaro is a well-evidenced, MHRA-authorised option that is genuinely accessible. Retatrutide is not yet a medicine in any regulatory sense. Seeking it out through informal online channels exposes you to serious and unquantifiable risks. If and when retatrutide reaches UK authorisation, it will be available through proper clinical routes — and that is the point at which it becomes worth a genuine conversation with your doctor. Until then, the responsible position is to work with what is available, evidenced and safe.

For a side-by-side look at how the current generation of weight loss treatments compare, see our Weight Loss Jabs and Pills Compared guide.

Frequently Asked Questions

Is retatrutide available in the UK?

No. Retatrutide has not received regulatory approval from the MHRA or any other medicines regulator. There is currently no MHRA-authorised retatrutide medicine available in the UK, privately or on the NHS. Products labelled as retatrutide sold online are not authorised medicines and cannot be verified for identity, purity or safety.

Did the retatrutide trials show better results than Mounjaro trials?

Phase 2 trial data published in the New England Journal of Medicine showed retatrutide producing mean weight loss of around 24.2% at 48 weeks at the highest dose, compared with around 20.9% for Mounjaro’s highest dose at 72 weeks in the SURMOUNT-1 Phase 3 trial. These were separate trials with different populations and durations — not a head-to-head comparison. Retatrutide’s Phase 3 TRIUMPH trials have since reported higher figures (28.3% at 80 weeks in TRIUMPH-1, in people without diabetes), but none compared it directly with Mounjaro.

Can I switch from Mounjaro to retatrutide when it becomes available?

Not yet, and not without a proper clinical process if retatrutide is eventually authorised. Any switch would be a decision made with a qualified prescriber based on your individual circumstances, the published evidence at that time and your response to your current treatment. It would not be automatic.

Why is retatrutide a triple agonist and does that make it better?

Retatrutide activates GLP-1, GIP and glucagon receptors simultaneously. Mounjaro activates GLP-1 and GIP only. The additional glucagon receptor activity is thought to increase energy expenditure as well as reducing appetite. Whether this translates into meaningfully better outcomes across diverse patient populations — and whether the additional mechanism affects the side effect profile — cannot be settled by the trials so far, because none has compared retatrutide directly with Mounjaro.

Who makes retatrutide?

Retatrutide is being developed by Eli Lilly, the same pharmaceutical company that makes Mounjaro (tirzepatide) and Zepbound.

This article is for information purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before starting any treatment.