Retatrutide vs Mounjaro: what's the difference?

Is retatrutide available in the UK the way Mounjaro is?

No. This is the single most important point, and it's worth being clear about before anything else. Mounjaro is a licensed prescription medicine in the UK, made by Eli Lilly, and can be dispensed through a regulated pharmacy after a clinical assessment. Retatrutide has not been licensed by the MHRA and remains an investigational medicine in clinical development.

That means you cannot legally obtain retatrutide from a UK pharmacy for weight management right now. Any seller offering it is operating outside the regulated system, and that carries real risk. If you're weighing up mounjaro vs retatrutide as things you might start today, the honest answer is that only one of them is an option.

A common misconception we hear is that a drug generating headlines in trials must already be buyable somewhere legitimate. It isn't, and that's not a loophole to chase. Licensing exists so that safety, manufacturing and dosing are all checked before a medicine reaches patients.

Mounjaro carries the Black Triangle symbol, meaning the MHRA is still gathering additional safety monitoring data even though it's approved. You can read the practical detail on our Mounjaro guide, which covers how the treatment works and who it suits. For now, retatrutide sits in a different category entirely: interesting, watched closely, but not something a UK prescriber can offer.

How do their mechanisms actually differ?

Mounjaro's active ingredient, tirzepatide, is a dual agonist. It works on two gut-hormone receptors, GIP and GLP-1, both involved in appetite and blood-sugar control. It slows how quickly the stomach empties and increases the feeling of fullness, which is why many people find they eat less without constant hunger. It's the only dual-agonist weight-loss medicine licensed in the UK, according to the NHS.

Retatrutide is being studied as a triple agonist. Alongside the GLP-1 and GIP pathways, it also targets the glucagon receptor, which is thought to influence how the body uses stored energy. In theory, adding that third pathway could change how much weight people lose and how the body responds. In practice, that theory is still being tested in trials.

The mechanical difference matters, but it doesn't automatically translate into a better real-world outcome. More receptors engaged is not the same as more benefit for any given person, and side-effect profiles can shift too. If the mechanism side interests you, our page comparing how Mounjaro and retatrutide are each designed goes deeper into how each works. What we can say firmly is that Mounjaro's mechanism has been assessed and licensed, while retatrutide's is still under investigation.

What does the trial evidence say about each?

Mounjaro has a substantial evidence base. In the SURMOUNT-1 trial, published in the New England Journal of Medicine and involving thousands of adults, tirzepatide produced average body-weight reductions of around 20 to 21% at the highest dose over 72 weeks. NICE has since recommended tirzepatide for eligible adults under TA1026. That's a mature, peer-reviewed picture.

Retatrutide's evidence is earlier-stage. Published phase 2 findings have reported notable average weight reductions, and larger phase 3 trials are ongoing. Early numbers have caught attention, but early-phase results in smaller groups don't carry the same weight as completed, regulator-reviewed programmes, and they can shift as bigger studies report.

You can see how the tirzepatide data holds up in our look at whether retatrutide is better. The verified data lives in the SURMOUNT-1 trial paper in the New England Journal of Medicine.

How are they taken, and does that change day to day?

Mounjaro is a once-weekly injection given under the skin using a pre-filled KwikPen, with each pen holding four weekly doses. UK strengths run from 2.5mg through to 15mg, and treatment begins at 2.5mg. That first dose isn't there to drive weight loss; its job is to let your system settle before the prescriber considers a step up, typically in four-week intervals.

You rotate injection sites across the abdomen, thigh or upper arm, and pens are kept in the fridge, often ending up in the door next to the milk. There's a limited window at room temperature, and you should always check the exact figure in the leaflet rather than rely on a general rule.

Retatrutide has been studied as a once-weekly injection too, following a titration approach broadly similar in principle. But because it isn't licensed, there's no approved UK dosing schedule, no pen you can pick up, and no leaflet to follow. That's a meaningful practical gap. With Mounjaro you have a defined, regulated routine; with retatrutide you'd be relying on trial protocols that aren't authorised for use outside a study. For a fuller picture of the licensed schedule, our main Mounjaro page walks through it without prescribing anything.

What about side effects and safety?

Both drugs sit in the same broad family, so the side-effect patterns overlap. With Mounjaro, the most common effects are gastrointestinal: feeling sick, an unsettled stomach, looser or firmer bowels, indigestion, tiredness and headaches. These tend to be most noticeable when you start or when the dose goes up, and for many people they ease within a couple of weeks. The NHS covers these in detail.

There's a rarer but serious risk worth naming. In January 2026 the MHRA issued a safety update flagging acute pancreatitis as an infrequent side effect of GLP-1 medicines: severe stomach pain that can reach through to the back, sometimes with vomiting, needs urgent medical attention. That applies to the licensed medicines this class contains.

Retatrutide's safety profile is still being characterised. Trials have reported gastrointestinal effects broadly in line with the class, and because the third receptor pathway is newer in humans, its full safety picture will only become clear as larger studies complete and a regulator reviews them. That's precisely the reassurance that licensing gives you with Mounjaro and can't yet give you with retatrutide. You can report any suspected side effect through the MHRA's Yellow Card scheme, and our shorter reta versus Mounjaro overview touches on the same ground more briefly.

Who can be prescribed Mounjaro in the UK today?

Because retatrutide isn't available, the only practical eligibility question is about Mounjaro. Privately, and in line with its licence, it may be considered for adults with a BMI of 30 or above, or 27 or above where there's a weight-related condition such as high blood pressure, high cholesterol, prediabetes or obstructive sleep apnoea. Lower thresholds can apply for some ethnic backgrounds under UK guidance.

On the NHS, access is much tighter and phased. NICE recommends tirzepatide under TA1026 for adults with a higher BMI plus weight-related conditions, and the rollout is being staged over time, so availability depends on where you live and your specific circumstances. Meeting a BMI figure never guarantees a prescription; a prescriber looks at your whole health picture.

At nume, a named prescriber reads your consultation answers rather than leaving the decision to software. That review happens the same day, and treatment is only approved where it's clinically suitable. It's the sort of judgement our clinical team makes routinely, and you can learn more about them on our clinician's profile. The staged NHS picture is set out by NICE in its tirzepatide appraisal. If you want to see where you stand, our page on applying for Mounjaro explains the next step plainly.

What does Mounjaro cost, and could you switch later?

Retatrutide has no UK price because it isn't sold here. For Mounjaro, private weight-loss treatment is priced per pen, per month, and varies by dose and provider. Eli Lilly raised Mounjaro's UK list price from 1 September 2025, with the highest dose's list price rising from around £122 to around £330 per four-week pen, and reported private prices since then typically range from roughly £149 to £375 a month.

We're honest about where we sit: nume isn't the cheapest, and that's a deliberate choice. One transparent price covers your consultation, prescription, treatment, free next-working-day tracked delivery and aftercare seven days a week, with no subscription and every repeat re-reviewed. Current pricing sits on our treatment page, and our cost breakdown puts it in context. For a prescription medicine, the lowest listing answers the wrong question anyway.

People often ask about moving to retatrutide down the line. Until it's licensed here, that isn't a switch you can make in the UK, and if you're curious about how the two treatments compare before making any decision, our page covering Zepbound vs Mounjaro is a useful place to understand how closely related medicines are assessed against each other. Our guide on switching from Mounjaro to retatrutide explains why that particular move isn't yet possible. Which suits you is a clinical decision our prescribers make with you, once they've assessed your full situation.

Related questions we've answered:

• Retatrutide or Mounjaro: which is better?
• Retatrutide vs tirzepatide compared
• Can I switch from tirzepatide to retatrutide?
• Tirzepatide vs Mounjaro: are they the same?