Tirzepatide or Retatrutide: Which Performs Better for Weight Loss?

What is retatrutide, and how does it differ from tirzepatide mechanically?

Tirzepatide, sold in the UK as Mounjaro, works by activating two gut-hormone receptors: GIP and GLP-1. Both receptors play a role in appetite regulation, slowing how quickly the stomach empties and signalling fullness to the brain. You can read a full overview on our tirzepatide information page.

Retatrutide takes that architecture one step further. It targets three receptors: GIP, GLP-1 and the glucagon receptor. Glucagon is primarily known for raising blood glucose, but activating the glucagon receptor also increases energy expenditure, the rate at which the body burns calories at rest. The hypothesis behind retatrutide is that adding this third pathway could produce greater fat loss than dual agonism alone.

Whether that hypothesis holds at scale is still being tested. Phase-two data published in the New England Journal of Medicine (2023) showed retatrutide reduced body weight by roughly 17% at the 4mg dose and around 24% at the 12mg dose over 48 weeks in adults without diabetes, figures that attracted considerable attention. But phase-two trials involve smaller groups, shorter follow-up and less diverse populations than phase-three programmes. They are hypothesis-generating, not confirmatory. For a fuller look at how these two compounds sit side by side, our page on retatrutide and Mounjaro covers the mechanism comparison in more depth.

Is retatrutide better than tirzepatide based on the trial numbers?

On raw percentage figures, the phase-two retatrutide results do exceed what tirzepatide produced in SURMOUNT-1. In that large, 72-week phase-three trial, tirzepatide at 15mg produced average weight reductions of around 20–21% in adults with obesity, as reported in the New England Journal of Medicine. Retatrutide's 12mg phase-two arm returned around 24% over a shorter 48-week window.

That looks decisive. It is not. Comparing a phase-two result at 48 weeks with a phase-three result at 72 weeks is methodologically problematic: different trial lengths, different patient populations, different protocols. Phase-three retatrutide trials are ongoing, and the final numbers could be higher, lower or similar to phase-two once tested in thousands of participants across diverse real-world-like conditions. Asking which is better between retatrutide or tirzepatide right now is a bit like comparing a prototype's test-track time with a production car's motorway performance: the conditions are too different for a fair verdict.

There is also no head-to-head trial data yet. SURMOUNT-5 compared tirzepatide directly with semaglutide 2.4mg (Wegovy), and tirzepatide produced greater average weight loss, but retatrutide was not part of that comparison. The question of whether retatrutide will outperform tirzepatide in a properly controlled head-to-head study remains genuinely open.

Can you access retatrutide in the UK, and what should you do in the meantime?

Retatrutide has not been approved by the MHRA or any other major regulatory authority. It is not legally available in the UK as a licensed medicine for weight management. Anything marketed as retatrutide for sale online in the UK would be unlicensed and unregulated, and the MHRA has consistently warned about the dangers of sourcing injectable weight-loss medicines outside the legitimate supply chain.

If you are curious about whether combining or switching between these compounds makes sense, our page on mixing tirzepatide and retatrutide addresses why that is not a safe or legal route either. A practical check worth doing right now: visit the GPhC pharmacy register and confirm that any online pharmacy you are considering is listed. It takes under a minute and is the single clearest signal of legitimate dispensing. Our Mounjaro page explains what tirzepatide treatment through a regulated UK service actually looks like in practice.

For people who are eligible today, tirzepatide is a well-evidenced, licensed option with a strong safety record from thousands of trial participants. The NICE appraisal of tirzepatide (TA1026) recommends it for adults with a BMI of 35 or above alongside at least one weight-related health condition, with lower thresholds applying for some ethnic backgrounds. Private prescribing through a registered pharmacy covers a broader group, including adults with a BMI from 27 upwards with a qualifying condition. Context on what treatment costs in the private market is set out on our Mounjaro price comparison page.

Retatrutide may one day receive approval, and if it does, the conversation will change. Until then, what is better, retatrutide or tirzepatide, is less a clinical question and more a regulatory one. The licensed medicine with real-world data wins by default, because it is the only one you can actually access safely. If you would like to find out whether tirzepatide is suitable for you, a prescriber at nume can review your case. Start your free consultation and get a same-day clinical review from a GPhC-registered Independent Prescriber.