Retatrutide: what to know about Eli Lilly’s next-generation weight loss injection

Retatrutide, also known in development as LY3437943, is an investigational medicine being developed by Eli Lilly. It is being studied as a once-weekly injection for adults with obesity or overweight and weight-related medical problems, type 2 diabetes and some complications linked with excess weight. [1,2]

The important point for UK readers is status. Retatrutide is not a licensed UK medicine. It is not something a UK prescriber can currently choose instead of Mounjaro or Wegovy, and it is not available through lawful routine supply routes. Lilly says retatrutide has not been approved by any regulatory agency and is legally available only to participants in Lilly clinical trials. [2]

Retatrutide is sometimes described online as 'Triple G', 'GLP-3' or 'reta'. These terms can be misleading. 'GLP-3' is not a scientific classification. Lilly describes the more accurate term as 'triple agonist', because it acts on three receptor pathways: GIP, GLP-1 and glucagon. [2]

Is Retatrutide available in the UK?

No. Retatrutide is not approved or legally available for routine use in the UK at the time of writing.

That means:

• it is not available on the NHS;
• it is not available through private weight-loss clinics;
• it is not available through legitimate online pharmacies;
• it does not have a confirmed UK brand name;
• it does not have a confirmed UK price;
• it does not have an authorised UK patient leaflet or Summary of Product Characteristics;
• it should not be purchased from online sellers, social media accounts, 'research peptide' suppliers, beauty clinics or informal sources.

The only lawful access described by Lilly is participation in Lilly clinical trials. [2]

How does Retatrutide work?

Retatrutide is part of the wider group of incretin-based medicines being studied for obesity and metabolic disease, but it is not simply another GLP-1 medicine.

Wegovy contains semaglutide, a GLP-1 receptor agonist. GLP-1 activity can help reduce appetite, increase fullness, slow gastric emptying and support glucose-dependent insulin secretion. [12]

Mounjaro contains tirzepatide, which activates both GIP and GLP-1 receptors. In the UK, Mounjaro is licensed for type 2 diabetes and weight management in eligible patients. [13]

Retatrutide adds a third receptor target: glucagon. Glucagon is often discussed as a hormone involved in raising blood sugar when levels are low, but glucagon receptor activity is also involved in energy balance, liver metabolism and fat metabolism. The clinical question is whether combining GIP, GLP-1 and glucagon activity can produce greater weight loss or broader metabolic effects without unacceptable tolerability or safety problems.

What do trial results show?

Retatrutide has produced large average weight reductions in clinical trials. The evidence is strongest for the published phase 2 obesity trial and Lilly's phase 3 TRIUMPH-1 obesity trial. Some phase 3 findings are currently company-reported or conference-presented rather than available as a full peer-reviewed obesity paper, so they should be treated as important but still developing evidence.

How much weight did people lose on Retatrutide?

In TRIUMPH-1, the highest 12mg dose produced an average body-weight reduction of 28.3% at 80 weeks. The average baseline weight in the trial was 112.7kg. That means the 12mg group lost an average of 31.9kg, or 70.3lb. [1]

The 9mg group lost an average of 25.9%, and the 4mg group lost an average of 19.0%. Lilly highlighted the 4mg dose because it was reached with a single dose-escalation step and had a lower observed discontinuation rate due to adverse events than placebo in TRIUMPH-1. [1]The highest-dose results are the reason retatrutide is attracting such attention. In TRIUMPH-1, 45.3% of people on 12mg achieved at least 30% body-weight reduction at 80 weeks, and 65.3% reached a BMI below 30. [1]

Those numbers should be read as trial averages, not individual promises. Some people lose more than average. Some lose less. Some stop treatment because of side effects, clinical advice, personal preference or lack of benefit. Trial participants also receive structured follow-up that does not mirror unsafe self-use from online sellers.

The 12mg TRIUMPH-1 result is impressive, but it is not a dosing instruction and it is not a reason to buy unlicensed retatrutide. Regulatory review exists to examine the full benefit-risk profile, manufacturing quality, approved dose escalation, contraindications, patient leaflet wording and monitoring requirements before a medicine enters routine care.

James Reynolds MPharm, DipClinPh, PgCert Derm, SCOPE, IP

Is retatrutide better than Mounjaro or Wegovy?

Retatrutide looks highly effective in trials, but it is not proven to be better for every patient in routine UK care.

The reported average weight loss with retatrutide is numerically higher than the headline results from established trials of semaglutide and tirzepatide. In STEP 1, semaglutide 2.4mg was associated with 14.9% average body-weight reduction at 68 weeks, compared with 2.4% on placebo. [14] In SURMOUNT-1, tirzepatide produced average weight reductions of 19.5% with 10mg and 20.9% with 15mg at 72 weeks, compared with 3.1% on placebo. [15]

That does not mean patients can make a simple league table. Trials can differ in participant characteristics, baseline BMI, diabetes status, support given alongside treatment, duration, dose escalation, missing-data handling and how discontinuations are analysed.

The careful conclusion is this: retatrutide may become an important future option, especially for people needing larger weight reductions or treatment for obesity-related complications. It has not replaced Mounjaro or Wegovy in UK care, and there is no authorised UK route to use it at present.

Retatrutide side effects: what is known so far?

The most common side effects reported in TRIUMPH-1 were gastrointestinal. Lilly reported nausea, diarrhoea, constipation and vomiting more often with retatrutide than placebo. [1]In TRIUMPH-1, nausea was reported in 28.6% of participants on 4mg, 38.4% on 9mg and 42.4% on 12mg, compared with 14.8% on placebo. Vomiting was reported in 10.6%, 22.8% and 25.3% respectively, compared with 4.8% on placebo. Diarrhoea and constipation were also common. [1]

Lilly also reported higher rates of dysesthesia, meaning unusual skin sensations such as tingling or altered sensation, and urinary tract infections in people taking retatrutide compared with placebo. These events were generally described as mild to moderate, and most participants continued treatment. [1]

Discontinuation due to adverse events increased with dose in TRIUMPH-1: 4.1% on 4mg, 6.9% on 9mg and 11.3% on 12mg, compared with 4.9% on placebo. [1]

Because it is not approved, it does not yet have a final UK patient leaflet. Patients should not assume that advice for Mounjaro or Wegovy applies exactly to retatrutide. The final safety profile, warnings, contraindications, medicine interactions and patient actions would only be confirmed if a regulator approves the medicine.

Retatrutide dosage, dosing and mixing questions

Search demand around retatrutide often includes phrases such as dosage, dosing, pen, source, where to get it and how to mix retatrutide. Those questions need a direct answer, but not a dosing guide.

The doses reported in trials are research doses used under trial protocols. For example, TRIUMPH-1 randomised participants to retatrutide 4mg, 9mg, 12mg or placebo. Participants started at 2mg once weekly and increased stepwise every four weeks until reaching the trial target dose. [1]

That is not an approved UK dosing schedule. It is not public prescribing advice. It should not be copied from a clinical trial into self-use.

This also applies to combinations promoted online, including combining 'reta' with cagrilintide, semaglutide, tirzepatide or other peptide products. These combinations are not an authorised treatment route and may increase risk, especially where the substance, dose, sterility and storage are unknown.

Can you buy Retatrutide online?

You should not buy retatrutide online. That includes sites using terms such as retatrutide peptide, research peptide, retatrutide pen, retatrutide vial, retatrutide UK source, retatrutide for sale, reta weight loss or similar wording.

A product can look medically packaged and still be unsafe. It may be fake, contaminated, incorrectly dosed, wrongly labelled, stored incorrectly or not retatrutide at all. The risks are not limited to wasting money. They include infection, severe vomiting or diarrhoea, dehydration, blood sugar problems, allergic reaction, contamination and harm from using an unknown substance alongside other medicines.

The MHRA has repeatedly reported enforcement action involving unlicensed weight-loss medicines. In October 2025, MHRA officers seized more than 2,000 unlicensed retatrutide and tirzepatide pens awaiting dispatch from an illicit facility in Northampton. [7] In February 2026, MHRA officers seized almost 2,000 doses of unauthorised weight-loss medicines from premises in Lincolnshire and Nottinghamshire, including retatrutide and tirzepatide. [8]

In May 2026, the MHRA reported its largest ever seizure of unlicensed weight-loss medicines, recovering around 12,000 doses from a country estate near Northampton, including retatrutide and tirzepatide. [9]

What to do if you have already used something sold as Retatrutide

This is a safety issue, not a moral judgement. Some people have already been exposed to unlicensed products because these products are being promoted online.

Do not use another dose unless a qualified clinician tells you to. Keep the packaging, pen, vial, label, order confirmation, batch number and screenshots if you have them, because they may help a clinician or regulator understand what you may have been exposed to.

You can report suspected fake medicines or suspicious online sellers to the MHRA. You can also report side effects and suspected fake products through the MHRA Yellow Card scheme. [10] [11]

How much will Retatrutide cost in the UK?

There is currently no confirmed UK price. Any website currently listing a UK retatrutide price or cost per month is not pricing an authorised UK medicine. It is selling, advertising or discussing an unlicensed product or speculative future supply. A low price is not a bargain if the product is illegal, unverified or unsafe.

If it is approved in the future, price will depend on the approved product, supply route, private prescribing market, any NHS commissioning decision and any NICE assessment. None of those details are confirmed at the time of writing.

What else is it being studied for?

Retatrutide is being studied as more than a general weight-loss medicine. Lilly says additional trials are evaluating its efficacy and safety in obesity or overweight with weight-related medical problems, type 2 diabetes, knee osteoarthritis pain, moderate-to-severe obstructive sleep apnoea, chronic low back pain, cardiovascular and renal outcomes, and metabolic dysfunction-associated steatotic liver disease. [2]

Type 2 diabetes

In TRANSCEND-T2D-1, retatrutide reduced HbA1c by up to 2.0% at 40 weeks in adults with type 2 diabetes and inadequate glycaemic control with diet and exercise alone. People taking 12mg retatrutide lost an average of 16.8% body weight. [4] [5]

This matters because people with type 2 diabetes often lose less weight in obesity-drug trials than people without diabetes. It does not mean retatrutide is approved for diabetes or weight management.

Knee osteoarthritis pain

In TRIUMPH-1 data presented by Lilly, retatrutide reduced WOMAC knee osteoarthritis pain scores by up to 4.3 points, or 73.1%, from a baseline of 6.0 in participants with knee osteoarthritis. [4]

That is clinically interesting because excess weight can increase mechanical loading on the knee and is linked with inflammatory and metabolic factors. It should not be read as proof that retatrutide is a general pain treatment or a replacement for orthopaedic assessment, physiotherapy, pain-management review or surgery where those are needed.

Obstructive sleep apnoea

Lilly reported that retatrutide reduced moderate-to-severe obstructive sleep apnoea severity by up to 36.1 events per hour, or 60.6%, using the apnoea-hypopnoea index. [4]

People with suspected sleep apnoea should not wait for future medicines. Loud snoring, witnessed pauses in breathing, waking with headaches, daytime sleepiness or falling asleep while driving should be discussed with a GP or sleep clinic because untreated sleep apnoea can affect driving safety, blood pressure and cardiovascular risk.

Fatty liver disease / MASLD

A phase 2a substudy published in Nature Medicine looked at retatrutide in metabolic dysfunction-associated steatotic liver disease, often shortened to MASLD. At 48 weeks, the 12mg dose was associated with an 86.0% relative reduction in liver fat, and 93% of participants taking 12mg achieved liver fat below 5%. [6]

Those findings are promising, but they do not make retatrutide an approved liver-disease treatment. Liver-fat reduction is an important signal. Liver outcomes also depend on inflammation, fibrosis, cardiometabolic risk and longer-term clinical endpoints.

When could retatrutide become available in the UK?

There is no confirmed UK availability date for retatrutide. A medicine does not become available because a trial result is positive. The manufacturer must submit evidence to regulators. Regulators then assess quality, safety and efficacy. If approved, the medicine would need a product label, manufacturing and supply arrangements, prescribing information and, for NHS use, health-technology assessment and commissioning decisions.

For UK patients, the practical questions are:

• Will the MHRA approve retatrutide?
• What indication will be approved?
• Who will be eligible?
• What dose range and step-up schedule will be authorised?
• What warnings and medicine-use restrictions will appear in the patient leaflet?
• What will it cost privately?
• Will NICE recommend it for NHS use?
• How will clinicians choose between retatrutide, Mounjaro, Wegovy and future oral medicines?

Until those answers exist, it should be treated as a future possibility, not a current treatment option.

Who might be eligible for Retatrutide if it is approved?

This cannot be answered definitively until a regulator approves the medicine and publishes the approved indication.

Based on the trial programme so far, retatrutide may be relevant to adults with obesity, adults with overweight and weight-related medical problems, adults with type 2 diabetes and adults with some obesity-related complications. [1,2,4]

If approved, it may be most relevant for people who need substantial weight reduction or who have significant obesity-related complications. That is a clinical inference, not a prescribing rule. Suitability would depend on the approved licence, medical history, current medicines, previous treatment response, diabetes status, family-planning context, gallbladder or pancreatic history, eating-disorder risk, mental health context, side-effect tolerance and clinician judgement.

What do we still not know about retatrutide?

There are still important unanswered questions:

• the final UK-approved indication, if approval is granted;
• the UK brand name, packaging and pen format;
• the approved dose range and step-up schedule;
• the final UK patient leaflet and Summary of Product Characteristics;
• the UK private price and any NHS access route;
• NICE's view on cost-effectiveness and eligibility;
• how it performs in real-world practice outside trial settings;
• whether higher average weight loss will be acceptable to broad patient groups from a side-effect and safety perspective;
• how clinicians will choose between retatrutide and established treatments;
• what long-term maintenance looks like over several years;
• what happens when treatment is stopped;
• whether direct head-to-head evidence against semaglutide or tirzepatide becomes available.

How should patients think about Retatrutide now?

Retatrutide may represent the next major step in medical weight management. The trial results are important, and the wider data in type 2 diabetes, knee osteoarthritis pain, sleep apnoea and liver fat suggest future obesity medicines may increasingly be judged by health outcomes beyond scale weight.

At the same time, the current safety message is simple. Retatrutide is not available in the UK as an approved treatment. Do not buy it online. Do not use a research peptide version. Do not follow mixing instructions, dosing charts or supplier advice. If weight is affecting your health now, speak to a qualified clinician about currently licensed treatment options and structured support.