Reta or Mounjaro: Which Does the Evidence Support?

What tirzepatide has proven at scale

The case for tirzepatide starts with SURMOUNT-1, the pivotal phase 3 trial published in the New England Journal of Medicine. It randomised 2,539 adults with obesity (without diabetes) and found average body-weight reductions of around 20–21% at the 15 mg dose over 72 weeks, alongside a meaningful placebo comparison. That is not a small study, and 72 weeks is long enough to see a genuine durability signal. NICE reviewed this evidence carefully before recommending tirzepatide in its technology appraisal TA1026, published in December 2024 and updated in September 2025. The recommendation for adults with a BMI of 35 or above and at least one weight-related comorbidity is grounded in that trial programme. Thresholds are lower for some ethnic backgrounds under UK guidance. The mechanism behind those results is tirzepatide's dual action: it activates both GIP and GLP-1 receptors, slowing gastric emptying, reducing appetite, and improving blood-sugar regulation simultaneously. You can read more about how the treatment works on our Mounjaro overview page.

What retatrutide has shown so far, and what that does and doesn't mean

Retatrutide adds a third mechanism to the two tirzepatide already targets: glucagon receptor agonism. In theory, activating the glucagon pathway alongside GIP and GLP-1 could drive greater energy expenditure and fat oxidation. Phase 2 data, published in 2023, reported weight reductions of up to roughly 24% at the highest dose over 48 weeks, figures that caught the attention of endocrinologists worldwide. The question the research community is now examining through phase 3 is whether those signals hold in larger, more diverse populations over longer periods, and what the safety profile looks like at scale. Phase 2 trials typically enrol a few hundred participants; SURMOUNT-1 enrolled more than 2,500. The difference in statistical power matters enormously when regulators decide whether to approve a medicine. Until retatrutide completes phase 3 and applies for a licence, there is no legitimate route to access it in the UK. Anyone offering it is not offering a licensed medicine. If you want to understand how reta compares with tirzepatide at the level of mechanism and trial data, we cover that in detail separately. The key differences between reta and Mounjaro go beyond mechanism, the regulatory gap is the defining one for anyone making a real treatment decision today.

Putting cost and access in context

Because retatrutide has no UK licence, there is no legal private prescription for it and no regulated price to quote. Tirzepatide, by contrast, is available through GPhC-registered pharmacies following a clinical consultation, and pricing context matters. Eli Lilly raised Mounjaro's UK list price from 1 September 2025, with the highest-dose pen's list price rising from around £122 to around £330; private market prices across providers have typically ranged roughly £149–£375 per month depending on dose. Our treatment overview explains what those costs usually include. If you are exploring all GLP-1 and related medicines currently in development, the landscape is moving fast. Our page on how reta sits alongside Mounjaro is a useful starting point for understanding where the two medicines stand relative to each other right now. What is stable is this: the evidence base for tirzepatide is mature and publicly available, it has passed MHRA scrutiny, and it is the only triple-or-dual agonist weight-loss medicine you can actually be prescribed in the UK right now.

Who is a suitable candidate for Mounjaro, and how a prescriber decides

Suitability for tirzepatide is a clinical question, not one that BMI alone settles. The licensed criteria cover adults with a BMI of 30 or above, or 27 or above with at least one weight-related condition such as high blood pressure, type 2 diabetes, high cholesterol, obstructive sleep apnoea, or cardiovascular disease. A prescriber also considers your full medical history, any medicines you are already taking, and whether you are pregnant, breastfeeding, or planning a pregnancy, none of which are situations where tirzepatide is recommended. Once clinically approved, tirzepatide treatment starts at 2.5 mg, a starting dose chosen to let your system adjust rather than to produce immediate therapeutic effect, and is titrated in steps by your prescriber. The pen itself is kept in the fridge door between doses, which most people find easy to build into a routine. If you are curious whether you would meet the criteria, our prescribers are the right people to ask. They review every consultation personally, a clinician reads your answers, not software. For anyone still weighing up the options, our dedicated page on retatrutide versus Mounjaro sets out the evidence side by side, and our page addressing whether reta is actually better than Mounjaro tackles that question honestly. You can read about the clinical team behind our service on the clinical team profile page. For broader questions about GLP-1 development and what is on the horizon, our FAQs cover the questions we hear most often. When you are ready to see whether tirzepatide is right for you, the place to start is a free consultation with our prescribing team.