Retatrutide is an experimental weekly injection from Eli Lilly that acts on three hormone receptors (GLP-1, GIP and glucagon). It isn't licensed in the UK or anywhere else, the MHRA says anyone selling it here is doing so illegally, and Lilly plans to submit it for US approval in early 2027. In the TRIUMPH-1 trial, published on 29 September 2026, people on the 12 mg dose lost 25.0% of their body weight on average over 80 weeks, against 3.9% on placebo. Mounjaro, which acts on two receptors and is licensed, averaged 20.9% at its top dose over 72 weeks in its own trial (SURMOUNT-1); the two have never been compared head to head.
Retatrutide is not licensed in the UK, and selling it here is illegal.
It's still in clinical trials and isn't approved anywhere in the world. The MHRA says anyone selling it in the UK is doing so illegally, and anything sold as retatrutide online, including vials labelled "for research only", has no checked contents, strength or sterility. The only legitimate way to take it is inside a clinical trial.
If you're on Mounjaro (tirzepatide) and you keep hearing about retatrutide, you're not alone. 2026 was the year its big trials reported, and the numbers are genuinely eye-catching. It's also not approved yet, which changes what's possible, and what's safe.
This article is for normal humans, not scientists. I'll explain what retatrutide is, how it compares to Mounjaro, what the 2026 trials found, what the side effects look like, why the online market is such a problem, and how to track your progress properly inside Healthcount.
Quick facts (30 seconds)
| Question | Quick answer |
|---|---|
| Is retatrutide approved in the UK? | No. It isn't licensed anywhere yet. Lilly plans to submit it for US approval in early 2027. |
| What is retatrutide? | A once-weekly injection that targets three hormone receptors (a "triple agonist"). |
| What is Mounjaro? | A once-weekly injection that targets two hormone receptors (a "dual agonist"). |
| Can you convert Mounjaro mg → retatrutide mg? | No official conversion exists. You can only compare trial averages (roughly), not "equivalent doses". |
| What did the 2026 trials find? | 25.0% average weight loss on 12 mg over 80 weeks in TRIUMPH-1, counting everyone who started (3.9% on placebo). |
| What's the biggest risk right now? | Unlicensed supply. The MHRA says anyone selling retatrutide in the UK is doing so illegally. |
If you're weighing up what's next, the most helpful thing you can do today is keep clean records
Healthcount is built for food, exercise, measurements, progress photos, and Mounjaro dosing logs.
Track Your ProgressWhat retatrutide is
Retatrutide is an investigational (not yet approved) medicine from Eli Lilly, given as a weekly injection and tested for obesity and a long list of related conditions.
It's usually described as a triple agonist because it activates three receptors:
- GLP-1 receptor
- GIP receptor
- Glucagon receptor
The first big signal came from a phase 2 trial in 2023: after 48 weeks, people on 12 mg had lost 24.2% of their body weight on average, against 2.1% on placebo (NEJM 2023). In 2026 the much larger phase 3 trials reported. They're covered further down, in the 2026 trial results.
What Mounjaro is
Mounjaro is the brand name for tirzepatide. In the UK, it has an official SmPC (Summary of Product Characteristics) that explains how it should be prescribed and used (Mounjaro SmPC).
Mounjaro is a dual agonist, meaning it activates two receptors:
- GLP-1 receptor
- GIP receptor
The UK SmPC also lays out how dosing usually works (start low and increase slowly): starting dose 2.5 mg once weekly, then move to 5 mg, then step up in 2.5 mg increments if needed (with at least 4 weeks at each step). In its main obesity trial, people on the top 15 mg dose lost 20.9% of their body weight on average over 72 weeks (SURMOUNT-1, NEJM 2022).
Tiny biology lesson: receptors and hormones
You don't need a biology degree for this. You just need the right mental picture.
What's a receptor?
A receptor is like a doorbell on a cell. When the right "finger" (a hormone or a medicine) presses it, the cell gets a message and reacts.
What's a hormone?
A hormone is a message chemical your body makes. Many hormones travel in your bloodstream (so yes, they're "in your blood"), but they aren't a separate liquid. They're molecules carried by the blood.
Why does this matter for weight and blood sugar?
Because drugs like Mounjaro and retatrutide are basically messages you inject, designed to press certain "doorbells" that affect:
- hunger and fullness
- how your body handles sugar after meals
- sometimes energy use
If you've ever wondered why these meds can change appetite so strongly, that's why.
Retatrutide vs Mounjaro: the big difference
Here's the headline:
- Mounjaro = dual agonist (2 targets)
- Retatrutide = triple agonist (3 targets)
So retatrutide has an "extra lever" (the glucagon receptor) on top of the GLP-1 and GIP effects.
Important: you can't compare them by "milligrams" alone
This is the part that saves you from a lot of confusion:
- 1 mg of Drug A is not automatically "stronger" or "weaker" than 1 mg of Drug B
- They're different molecules, hitting different receptors, with different potencies and different dose-response curves
- So there is no official Mounjaro-to-retatrutide conversion chart
- What you can do is compare trial averages at specific timepoints, with a big "rough comparison" label
"What dose of retatrutide equals my Mounjaro dose?"
Let's answer this the way your readers are actually asking it.
The honest answer (the one you can trust)
There is no evidence‑based, safe, "same impact" conversion from Mounjaro mg to retatrutide mg — especially because retatrutide isn't approved yet.
What we can do is compare published trial results at different doses and see what looks similar on average (not person‑to‑person).
A clear, rough comparison table (trial averages, not a dosing guide)
Not a conversion chart. This is only to help you understand why "mg" doesn't translate cleanly. Different studies, different timeframes, different people.
| Medicine | Dose group | Timepoint | Average weight change |
|---|---|---|---|
| Mounjaro (tirzepatide) | 5 mg weekly | 72 weeks | −16.0% |
| Mounjaro (tirzepatide) | 10 mg weekly | 72 weeks | −21.4% |
| Mounjaro (tirzepatide) | 15 mg weekly | 72 weeks | −22.5% |
| Retatrutide (investigational) | 1 mg weekly | 48 weeks | −8.7% |
| Retatrutide (investigational) | 4 mg weekly (combined 4 mg groups) | 48 weeks | −17.1% |
| Retatrutide (investigational) | 8 mg weekly (combined 8 mg groups) | 48 weeks | −22.8% |
| Retatrutide (investigational) | 12 mg weekly | 48 weeks | −24.2% |
Let's explain:
- In trials, retatrutide 4 mg at 48 weeks landed around the same average weight‑loss range as Mounjaro 5 mg at 72 weeks — but that does not make them equivalent doses.
- And for higher doses: retatrutide 8–12 mg shows higher average loss at 48 weeks, while Mounjaro 10–15 mg shows strong results at 72 weeks.
Will you have to "dose ladder" again if you switch from Mounjaro to Retatrutide?
Very likely, yes.
Why? Because the slow step‑up is how these medicines reduce side effects, especially nausea and vomiting.
- Mounjaro's UK SmPC describes step‑ups in 2.5 mg increments after at least 4 weeks at a dose.
- In the Phase 2 retatrutide trial, some groups used a lower starting dose (2 mg vs 4 mg) and the study notes side effects were "partially mitigated" with a lower starting dose.
To be on the safe side: Even if you've used Mounjaro before, a new medicine will usually mean a fresh titration schedule (if and when it's approved), because tolerability matters.
What retatrutide is being studied for
Retatrutide isn't just being studied for weight loss. Lilly's phase 3 TRIUMPH programme covers several conditions linked to obesity, and four of those trials had reported results by October 2026.
Here's the list that's been publicly described:
| Category | Examples in the Phase 3 programme |
|---|---|
| Weight management | Obesity, overweight with comorbidities |
| Metabolic health | Type 2 diabetes |
| Pain and mobility | Knee osteoarthritis; chronic low back pain |
| Sleep / breathing | Obstructive sleep apnoea |
| Heart / kidneys | Cardiovascular outcomes; renal outcomes |
| Liver | Metabolic dysfunction-associated steatotic liver disease (MASLD) |
The 2026 trial results
Each trial reports two numbers, and it helps to know which is which. One counts everyone who started, including people who stopped (the "treatment-regimen" result). The other estimates what would have happened if everyone had stayed on the drug (the "efficacy" result). The second is bigger, and it's the one most headlines use. We give both where we have them.
TRIUMPH-1: obesity or overweight, without diabetes
2,339 adults, 80 weeks, three doses against a placebo. Lilly announced the headline results on 21 May 2026, and the full paper came out in the New England Journal of Medicine on 29 September 2026 (NEJM).
| Weekly dose | Average weight lost, counting everyone who started | If everyone had stayed on it |
|---|---|---|
| Retatrutide 4 mg | 17.6% | 19.0% |
| Retatrutide 9 mg | 23.7% | 25.9% |
| Retatrutide 12 mg | 25.0% | 28.3% |
| Placebo | 3.9% | 2.2% |
On 12 mg, 45.3% of people lost at least 30% of their body weight, against 0.5% on placebo, on the "if everyone had stayed on it" basis (Lilly, ADA 2026). You may also see 30.3% quoted. That came from a 104-week extension in 532 people with a starting BMI of 35 or more who finished the main trial on their target dose, not from the whole trial.
TRIUMPH-2: obesity or overweight with type 2 diabetes
1,152 adults, 80 weeks, published in The Lancet on 29 September 2026. Counting everyone who started, people lost 11.9% on 4 mg, 16.8% on 9 mg and 18.8% on 12 mg, against 5.1% on placebo. If everyone had stayed on it, 12 mg gave 20.8% (The Lancet) (Pharmacy Times). HbA1c, the long-term blood sugar measure, fell by about 1.5 points on 12 mg from an average starting level of 7.7%, on the "if everyone had stayed on it" basis (Lilly, 29 September 2026). People with type 2 diabetes usually lose less weight on these medicines than people without it, which is why these numbers are lower.
TRIUMPH-3: severe obesity and heart disease
1,949 adults with a BMI of 35 or more and established cardiovascular disease. Lilly reported on 23 July 2026 that people lost 21.6% on 9 mg and 22.6% on 12 mg over 80 weeks, against 3.2% on placebo, if everyone had stayed on treatment (Lilly). Heart events were tracked, but the differences between the groups weren't statistically significant either way. There's no journal paper yet.
TRIUMPH-4: knee osteoarthritis
445 adults with obesity and knee osteoarthritis, reported on 11 December 2025. On 12 mg, people lost 23.7% of their body weight over 68 weeks counting everyone who started (28.7% if everyone had stayed on it), against 4.6% on placebo. Knee pain scores fell by about three-quarters on both doses, against about 40% on placebo, if everyone had stayed on it (Lilly).
Published trial data
Average weight lost on retatrutide 12 mg, counting everyone who started
Three retatrutide trials, with Mounjaro's main trial for context. Separate trials, not head to head.
- The medicine
- Placebo in the same trial
| Trial group | Average weight lost |
|---|---|
| Retatrutide 12 mg, TRIUMPH-1 | 25.0% |
| Placebo, TRIUMPH-1 | 3.9% |
| Retatrutide 12 mg, TRIUMPH-2 | 18.8% |
| Placebo, TRIUMPH-2 | 5.1% |
| Retatrutide 12 mg, TRIUMPH-4 | 23.7% |
| Placebo, TRIUMPH-4 | 4.6% |
| Mounjaro 15 mg, SURMOUNT-1 | 20.9% |
| Placebo, SURMOUNT-1 | 3.1% |
Where retatrutide stands in the UK
Is retatrutide available in the UK right now?
No. It isn't licensed here, or anywhere else in the world. Outside authorised clinical trials, the MHRA says any product sold in the UK that claims to contain retatrutide is likely to be illegal and potentially dangerous (MHRA).
When might it be licensed?
Lilly says it plans to submit retatrutide for US approval in the first quarter of 2027, once its manufacturing data package is complete (Lilly, 23 July 2026). No UK application has been announced. A UK licence would need its own MHRA review, and NHS use would then need a NICE appraisal on top of that.
The short version: the trials have reported and they're strong, but a UK prescription is still some way off. Drug timelines slip often, so treat any UK date you see quoted as a guess.
Retatrutide side effects: what the trials found
Most of the side effects people talk about are stomach and gut related, and they're worst while the dose is going up. That's not because the drug is "bad". These hormone pathways run through appetite and digestion. Retatrutide also has a couple of side effects you see less often with Mounjaro, and they're worth knowing about before you read anyone's glowing review.
Published data
Side effects in TRIUMPH-1, by dose
Share of people who had each one at least once over 80 weeks
- Placebo (no medicine)
- 4 mg
- 9 mg
- 12 mg
| Side effect | Placebo | 4 mg | 9 mg | 12 mg |
|---|---|---|---|---|
| Feeling sick (nausea) | 14.8% | 28.6% | 38.4% | 42.4% |
| Diarrhoea | 13.5% | 25.2% | 34.1% | 32% |
| Constipation | 10.9% | 23.8% | 25.9% | 26.1% |
| Being sick (vomiting) | 4.8% | 10.6% | 22.8% | 25.3% |
| Less appetite | 5.8% | 12.8% | 13.4% | 16.5% |
| Odd skin sensations | 0.9% | 5.1% | 12.3% | 12.5% |
Dysaesthesia: the skin side effect
Dysaesthesia means odd, unpleasant skin sensations, such as tingling, burning, or skin that feels sore to the touch. In TRIUMPH-1 it affected 12.5% of people on 12 mg, against 0.9% on placebo. In the knee trial, TRIUMPH-4, it was 20.9% on 12 mg (Lilly). A similar skin effect (skin sensitivity) was reported in the 2023 phase 2 trial, in 7% of people on retatrutide against 1% on placebo (NEJM 2023).
Heart rate and blood pressure
In the phase 2 trial, heart rate rose with the dose, peaking around week 24 at about 9 beats a minute above the starting level on 12 mg, then falling back (NEJM 2023). In TRIUMPH-1, low blood pressure events were more common on retatrutide (8.8% on 12 mg against 0.9% on placebo), especially in people already taking blood pressure medicines (Lilly, ADA 2026).
How many people stopped?
In TRIUMPH-1, 11.3% of people on 12 mg stopped because of side effects, against 4.9% on placebo (4.1% on 4 mg and 6.9% on 9 mg). Serious adverse events were reported in 10.5% on 12 mg, against 5.5% on placebo (Lilly, ADA 2026). In TRIUMPH-4, 18.2% on 12 mg stopped, and Lilly said that included people who stopped because they felt they'd lost too much weight.
| Type | Examples | What to do with this info |
|---|---|---|
| Stomach and gut (most common) | nausea, diarrhoea, constipation, vomiting, lower appetite | Expect it to be most noticeable during dose increases. Tell the trial team or your doctor if it's persistent or severe. |
| Skin and nerve sensations | dysaesthesia (tingling, burning, sore skin) | Don't ignore new or worrying symptoms. Get medical advice. |
| Heart and circulation | a faster heartbeat, dizziness from low blood pressure | Tell the trial team or your doctor, especially if you take blood pressure medicines. If you faint or have chest pain, call 999. |
| General | feeling very full, eating much less | Track what you eat so you can spot when a small appetite tips into too little food. |
If you're in a trial, report side effects to the trial team. If you've taken a product sold as retatrutide and feel unwell, call NHS 111, or 999 (or go to A&E) if you have severe stomach pain that won't go away, yellow skin or eyes, chest pain, fainting, or you can't keep fluids down. Tell them exactly what you took. You can also report it on the MHRA's Yellow Card website, which takes reports of suspected side effects and of fake or unlicensed medicines.
Retatrutide on sale in the UK is illegal
"Don't be tempted by the online hype around retatrutide. It has not been authorised for use in the UK, meaning anyone selling this product is doing so illegally."
That's the MHRA, in July 2026 (GOV.UK). Products sold online as retatrutide, often labelled "for research only" or "not for human consumption", come from outside any regulated supply chain. Nobody has checked what's in the vial, how strong it is, or whether it's sterile.
This isn't a theoretical risk. In October 2025 the MHRA seized more than 2,000 unlicensed retatrutide and tirzepatide pens from an illegal UK facility (MHRA), and in May 2026 it seized around 12,000 doses of unlicensed weight-loss medicines, including retatrutide, and arrested two men (MHRA). In June 2026, health officials in Victoria, Australia, warned about six people with acute liver toxicity possibly linked to a contaminant in products labelled retatrutide (Victoria Department of Health).
Lilly, which makes the real drug, says no medicine containing retatrutide has been approved for human use anywhere. On 12 August 2026 it filed six lawsuits against US sellers and said it had referred more than 200 individuals and companies to the FDA and other authorities (Eli Lilly). In the US, the FDA says retatrutide can't legally be used in compounding, and it has sent warning letters to companies selling it (FDA).
The simple truth
If it isn't coming from a clinical trial, you cannot reliably know:
- what's in it,
- how strong it is,
- whether it was stored correctly,
- whether it's sterile.
That's a big deal for anything you inject. If you're interested in retatrutide, the legitimate route is a clinical trial, and your GP or the trial listings can tell you what's recruiting.
What to track in Healthcount (so this doesn't become guesswork)
If you only do one thing after reading this, do this: track the basics consistently.
Not because you need to "obsess". Because your memory lies to you. (Mine does too.)
Here's a simple routine that fits real life.
The Healthcount tracking plan (simple and sustainable)
| What to track in Healthcount | Best frequency | Why it helps |
|---|---|---|
| Weight | Daily or weekly (weigh daily if you can, then look at your weekly average) | Daily weight jumps around (water, salt, sleep). Weekly averages show the real trend. |
| Measurements (waist, hips, etc.) | Monthly | Tape measures change slower than the scale, but they're brilliant for long-term progress. |
| Progress photos | Weekly or monthly | Weekly if you like detail; monthly if you prefer less pressure. Consistency beats perfection. |
| Food | Daily (rough is fine) | Helps you spot patterns (weekends, snacks, drinks) without turning food into maths homework. |
| Exercise | Each workout | Lets you connect progress to behaviour changes ("I actually moved more this month"). |
| Dosing | Every dose | If something changes, you can line it up with the timeline. Learn about weekly injection timing. |
If you're already putting the effort in, you might as well get the benefit
Use Healthcount to build a clear "week by week" story you can actually trust.
Start TrackingFAQs
Is retatrutide licensed in the UK?
No. It isn't licensed in the UK or anywhere else in the world. Lilly plans to submit it for US approval in the first quarter of 2027. The MHRA says anyone selling retatrutide in the UK is doing so illegally.
How much weight did people lose on retatrutide?
In TRIUMPH-1, published in the New England Journal of Medicine on 29 September 2026, people on 12 mg lost 25.0% of their body weight on average over 80 weeks, against 3.9% on placebo, counting everyone who started. Assuming everyone kept taking it, the figure was 28.3%. In people with type 2 diabetes (TRIUMPH-2), 12 mg gave 18.8%, or 20.8% assuming everyone kept taking it.
What are the side effects of retatrutide?
Mostly stomach and gut effects. In TRIUMPH-1, 42.4% of people on 12 mg felt sick at some point (14.8% on placebo), 32.0% had diarrhoea, 26.1% constipation and 25.3% vomiting. 12.5% had dysaesthesia, odd skin sensations such as tingling or sensitivity, against 0.9% on placebo. 11.3% on 12 mg stopped because of side effects, against 4.9% on placebo. Suspected side effects, including from products sold as retatrutide, can be reported on the MHRA Yellow Card website.
Is it legal to buy retatrutide in the UK?
It's illegal to sell it here. Retatrutide isn't authorised for use in the UK, so the MHRA says anyone selling it is doing so illegally. Products sold as retatrutide online, including those labelled "for research only", have no checked contents, strength or sterility.
Can I switch to retatrutide right now in the UK?
No. It isn't licensed, so it can't be prescribed, and the MHRA says anyone selling it is doing so illegally. The only legitimate way to take it is in an authorised clinical trial.
Is retatrutide "better" than Mounjaro?
Its phase 3 averages are higher than Mounjaro's were in Mounjaro's own trials, but they've never been tested head to head. More people stopped because of side effects, too: 11.3% on retatrutide 12 mg in TRIUMPH-1, against 6.2% on tirzepatide 15 mg in SURMOUNT-1, in separate trials. "Better" also depends on tolerability, long-term safety and what you personally need, and retatrutide doesn't have years of real-world use behind it yet.
If I'm doing well on Mounjaro, should I chase what's next?
Not automatically. If you're losing weight, feeling okay, and your health markers are improving, that's already a win. Newer isn't always better for you. Sometimes it's just newer.
Do I have to titrate again if I switch medicines?
Very likely, yes. Both Mounjaro's prescribing info and retatrutide trial design reflect the idea of starting lower and stepping up to reduce side effects.
View sources (19)
- Jastreboff AM, et al. Retatrutide for the treatment of obesity (TRIUMPH-1). New England Journal of Medicine, published online 29 September 2026
- Lilly: retatrutide TRIUMPH-1 phase 3 topline results, 21 May 2026
- TRIUMPH-1 results presented at the ADA Scientific Sessions, Eli Lilly, June 2026 (PDF)
- Retatrutide in adults with obesity and type 2 diabetes (TRIUMPH-2). The Lancet, published online 29 September 2026
- Retatrutide cuts weight up to 18.8% in adults with type 2 diabetes and obesity, Pharmacy Times, 30 September 2026
- Lilly: retatrutide TRIUMPH-2 results in obesity and type 2 diabetes, 29 September 2026
- Lilly: retatrutide successful in two additional phase 3 trials (TRIUMPH-2 and TRIUMPH-3), 23 July 2026
- Lilly: retatrutide TRIUMPH-4 results in knee osteoarthritis, 11 December 2025
- Jastreboff AM, et al. Triple-hormone-receptor agonist retatrutide for obesity, a phase 2 trial. New England Journal of Medicine, 2023
- No summer shortcut for safe weight loss, MHRA, GOV.UK, 24 July 2026
- MHRA smashes major illicit weight loss medicine production facility in record seizure, GOV.UK, October 2025
- Two arrested during the MHRA's largest ever seizure of unlicensed weight loss medicines, GOV.UK, 29 May 2026
- Lilly calls on online platforms, payment companies and regulators to shut down the illegal retatrutide black market, Eli Lilly, 12 August 2026
- FDA's concerns with unapproved GLP-1 drugs used for weight loss, US Food and Drug Administration
- Toxicity linked to unapproved peptide product labelled Retatrutide, Victoria Department of Health, June 2026
- Mounjaro, UK Summary of Product Characteristics, electronic medicines compendium
- Jastreboff AM, et al. Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). New England Journal of Medicine, 2022
- Retatrutide phase 2 obesity trial, PubMed summary
- Yellow Card: report a suspected side effect or a fake medicine, MHRA
