Three generations of one idea
All three medicines build on the same principle: mimic the gut hormones that regulate appetite and blood sugar. Wegovy (semaglutide) activates the GLP-1 receptor. Mounjaro (tirzepatide) activates GLP-1 and GIP. Retatrutide activates GLP-1, GIP and the glucagon receptor — the third signal being the novel one, thought to increase energy expenditure rather than just reducing intake.
Each added receptor has, so far, translated into larger average weight loss in trials. That pattern is real — but it says nothing about what is licensed, quality-assured and safe to use today.
Approval status: the difference that matters most
Wegovy and Mounjaro hold full MHRA licences for weight management, are stocked by regulated UK pharmacies, and are prescribed daily across the country. Retatrutide holds no licence anywhere in the world. It is still in phase 3 trials, and every "retatrutide" product sold outside those trials is unlicensed and unverified — the risks are covered in our guide to where you can get retatrutide in the UK.
Comparing the three as if they were parallel options is therefore misleading. Two are medicines; one is still an experiment.
What the trial results show
In their pivotal licensing trials, semaglutide produced average weight loss of roughly 15% over 68 weeks, and tirzepatide around 20% at the highest dose over 72 weeks. Retatrutide's published phase 2 data reported around 24% at the highest dose over just 48 weeks — the reason for the excitement. Phase 3 results will show whether that holds across larger populations and longer follow-up.
Averages hide variation: in every trial some people lost considerably more, some considerably less, and results depended on staying on treatment alongside diet and activity changes. "Which medicine suits this patient" is a different question from "which trial average is biggest" — and it is the first question a prescriber actually answers.
Cost and access: the practical comparison
Retatrutide has no UK price because it has no UK product — anything quoted online is a grey-market figure for an unverified vial, not a medicine's price. Wegovy and Mounjaro, by contrast, have transparent private pricing through regulated pharmacies, published dose schedules, and defined NHS eligibility routes through specialist weight-management services for patients who meet NICE criteria.
Access works differently too. A licensed medicine comes with a consultation, an eligibility assessment, dose-escalation support and follow-up — infrastructure that exists because the regulator requires treatment to be supervised. The grey market's "access" is a checkout page. When people compare the three medicines, they often compare trial percentages; the more consequential comparison is between a supervised treatment pathway and an unsupervised parcel.
Side effects and tolerability
All three share the class profile: gastrointestinal effects — nausea, vomiting, constipation, diarrhoea — concentrated during dose escalation and usually settling with time. Tolerability differences between them are modest in trial data, and individual response varies more than the averages do. Our guide to GLP-1 side effects covers management in practice. Retatrutide's long-term safety remains an open question by definition: the trials designed to answer it have not finished.
Which can you actually get in the UK?
Wegovy and Mounjaro: yes, by private prescription after a clinical consultation (and on the NHS within specialist services for eligible patients). Retatrutide: only inside a clinical trial. There is also a third licensed format now — the once-daily tablet Foundayo (orforglipron) — for people who prefer to avoid injections altogether.
If you are weighing these options, our weight-management guides go deeper on each, and a UK-registered clinician can tell you which licensed option fits your health picture. The consultation is free, carries no obligation, and replaces weeks of comparison reading with an answer that is actually about you.
What about type 2 diabetes?
All three molecules matter to diabetes care as well as weight management, but their positions differ. Semaglutide has long been licensed for type 2 diabetes as Ozempic, at diabetes-specific doses distinct from Wegovy's. Tirzepatide is licensed for both type 2 diabetes and weight management, and its dual GLP-1/GIP action has shown particularly strong glycaemic results — one reason prescribers often favour it for patients living with both conditions. Retatrutide is being trialled in type 2 diabetes too, with published early data showing meaningful HbA1c reductions, but the same rule applies: investigational everywhere, licensed nowhere.
If you have diabetes, the choice between these medicines involves your existing diabetes treatment, hypoglycaemia risk with certain combinations, and monitoring — decisions that belong with your diabetes team or prescriber rather than a comparison article. What the comparison can tell you is that the licensed options already cover this ground well.
Format and dosing in practice
All three are once-weekly subcutaneous injections, and all three use stepwise dose escalation: treatment starts low and increases over weeks to months, which is how the gastrointestinal side effects are kept manageable. Wegovy and Mounjaro come as pre-filled pens designed for self-injection at home after simple training; retatrutide in trials is administered within a monitored protocol. For people who want this class of treatment without needles at all, the newly approved once-daily tablet Foundayo (orforglipron) now exists as a licensed alternative — a genuinely new option this year.
How a clinician actually chooses between them
Prescribers weigh factors that never appear in headline comparisons: your BMI and weight-related conditions; diabetes status (tirzepatide's dual action is particularly relevant there); other medicines and interactions; previous experience with this class; injection confidence; supply availability; and cost, since private treatment is ongoing. Trial averages set expectations, but tolerability in your body sets reality — some people do better on one medicine than another for reasons averages cannot predict, and prescribers switch treatments when the first choice does not suit.
Supply is part of the decision too. The weight-management injectables have seen periods of constrained stock in the UK as global demand surged, and prescribers sometimes steer between equivalent options simply on dependable availability — another factor a league table never shows, and another reason the conversation belongs with someone who can see the current picture.
That is also why "which is strongest" is the wrong first question. The strongest medicine you cannot tolerate, cannot access or should not take is worth less than the right one prescribed properly. The comparison that matters is between you-on-option-A and you-on-option-B — a conversation, not a league table.