Retatrutide UK: The Triple Hormone Weight Loss Injection Explained (GLP-1 + GIP + Glucagon)

Retatrutide is one of the most talked about next generation weight loss injections because it’s designed to target three hormone pathways linked to appetite, blood sugar control and energy use (unlike Wegovy and Mounjaro, which target one or two).

But there’s a crucial catch: retatrutide is currently not available as a licensed treatment in the UK.

Is retatrutide available in the UK?

No. Retatrutide is still in clinical development and is not approved for UK use. The MHRA has warned that, outside authorised clinical trials, products being sold in the UK claiming to contain retatrutide are likely to be illegal and potentially dangerous.

If you’ve seen “retatrutide pens” advertised via social media, Telegram or WhatsApp groups, beauty salons, or “research peptide” websites, treat it as a major red flag. The MHRA has taken enforcement action against illicit supply chains, including a major seizure of unlicensed weight loss pens in October 2025.

If you want to be ready early (without taking risks): join our Retatrutide UK updates list, and we’ll share credible trial and regulatory milestones as they happen.

What is retatrutide?

Retatrutide (also known as LY3437943) is an investigational medicine being studied for obesity and metabolic disease. It’s designed to activate three receptors involved in weight regulation:

• GLP-1 (glucagon-like peptide-1)

• GIP (glucose-dependent insulinotropic polypeptide)

• Glucagon

This “triple agonist” design is why retatrutide is often described as a triple-hormone weight loss injection.

Is retatrutide a peptide? What is a peptide?

Yes, retatrutide is a synthetic peptide medicine. In simple terms:

• A peptide is a short chain of amino acids (the same building blocks that make proteins).

• Many hormones in the body are peptide based signals, which is why peptide medicines are common in metabolic health.

Why are peptide medicines usually injections?

Peptide medicines are often given by injection because peptides can be broken down in the stomach and absorbed poorly if swallowed,  injection is a more reliable way to deliver the medicine.

Does “peptide” mean “safe”?

Not automatically. “Peptide” describes the type of molecule, not the quality of what you’re being sold. The MHRA has repeatedly warned about fake or unlicensed pens being sold via unregulated routes which can mean the contents, dose, sterility and storage can’t be trusted.

Free Weight-Loss Consultation (Pharmacist-Led)

Not sure whether Wegovy or Mounjaro is right for you — or how to lose weight safely and sustainably?

Book a free 15 minute consultation with our clinical team at Puri Pharmacy. We’ll review your goals, medical history, and eligibility, and explain your safest options (including support plans).

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Retatrutide trial results so far

Phase 2 study (48 weeks)

In a 48 week clinical study, people taking retatrutide lost significant weight, especially at higher doses.

Average weight loss after 48 weeks

• Highest dose: around 24% of the starting body weight

• Mid dose: around 23%

• Lower dose: around 17%

• Placebo (dummy injection): around 2%

What does 24% actually mean?

If someone started at 100 kg, a 24% reduction is about 24 kg lost (on average).
If someone started at 120 kg, that’s about 29 kg lost.

How many people lost a meaningful amount of weight?

At the higher doses, results were very consistent:

• All participants lost at least 5% of their body weight

• Around 3 in 4 lost 15% or more

• With placebo, only around 1 in 4 lost 5% or more

Phase 3 update (TRIUMPH-4, 68 weeks, topline results)

In December 2025, Eli Lilly shared early (“topline”) results from a larger phase 3 study (TRIUMPH-4). In that study, the highest dose group reported an average weight loss of 28.7% at 68 weeks.

What does “topline” mean?

It’s an early summary released before the full scientific paper is published. The full details (including safety and how results were calculated) matter most for regulators.

How retatrutide works: the triple hormone mechanism

Retatrutide’s potential comes from combining three effects that influence appetite, glucose regulation and energy use.

1) GLP-1: appetite control + slower digestion

GLP-1 activation (the pathway used by semaglutide/Wegovy) helps people feel fuller sooner, reduces appetite, and slows gastric emptying often reducing snacking and lowering overall intake.

2) GIP: metabolic support + enhanced satiety signalling

GIP is part of what makes tirzepatide/Mounjaro a dual-acting medicine. In combination with GLP-1, it appears to strengthen appetite regulation and metabolic control for many people.

3) Glucagon: energy use + fat mobilisation (the differentiator)

This is the “third lever”. Glucagon signalling is associated with changes in energy expenditure and fuel use. Retatrutide’s glucagon activity is one reason researchers think it may drive deeper fat loss for some people than single- or dual-pathway medicines.

Retatrutide vs Wegovy vs Mounjaro: what’s the difference?

It’s tempting to compare these injections like a simple “league table”, but the studies were done in different groups of people, over different time periods, and with different study designs. So the numbers below are a useful guide, not a perfect head-to-head comparison.

The simplest way to think about it

• Wegovy works on 1 pathway (GLP-1)

• Mounjaro works on 2 pathways (GLP-1 + GIP)

• Retatrutide (still being studied) works on 3 pathways (GLP-1 + GIP + glucagon)

More pathways may mean stronger appetite and metabolic effects, which is why retatrutide is getting so much attention.

What did people lose in the headline studies? (average results)

Wegovy (semaglutide) — GLP-1 only

In the STEP-1 study (68 weeks), average weight change was about:

• –14.9% with Wegovy

• –2.4% with placebo (dummy injection)

What that looks like:

If you start at 100 kg, –14.9% is about 15 kg lost on average.

Mounjaro (tirzepatide) — GLP-1 + GIP

In SURMOUNT-1 (72 weeks), average weight loss at the highest dose was often quoted around:

• ~–22.5% (highest dose)

What that looks like:
If you start at 100 kg, ~–22.5% is about 22–23 kg lost on average.

Retatrutide (investigational) — GLP-1 + GIP + glucagon

Retatrutide is still in trials (not approved in the UK). So far, studies have reported:

• Phase 2 (48 weeks): up to –24.2% average weight loss

• Phase 3 TRIUMPH-4 topline (68 weeks): –28.7% average weight loss (press release)

What that looks like:
If you start at 100 kg:

• –24.2% ≈ 24 kg average loss

• –28.7% ≈ 29 kg average loss

Side effects and safety signals (what we know so far)

Phase 2 (peer-reviewed)

In the phase 2 trial, the most common adverse events were gastrointestinal and dose-related, typically mild to moderate. The trial also reported dose-dependent increases in heart rate that peaked at 24 weeks and declined thereafter.

Phase 3 TRIUMPH-4 (topline)

In Lilly’s topline summary, the most common adverse events included nausea, diarrhoea, constipation and vomiting (typical of incretin-based therapies). Discontinuation due to adverse events was reported as higher at the top dose than placebo.

Who might retatrutide be for (if it’s approved)?

Phase 2 included adults with

• BMI ≥30, or

• BMI 27 to <30 plus at least one weight-related condition

If retatrutide is eventually authorised in the UK, the final MHRA indication will define:

• eligibility criteria

• dosing schedules

• monitoring requirements

• contraindications and cautions

When could retatrutide arrive in the UK?

There is no confirmed UK launch date.

Retatrutide is still in phase 3 development, and regulators would need to review full data before any authorisation decision. Lilly has indicated multiple phase 3 readouts are expected in 2026.

If you want to be ready early (without taking risks), the safest approach is:

• stay updated on credible trial/regulatory milestones

• avoid unlicensed supply routes

• focus on sustainable foundations (protein, resistance training, sleep, side-effect management, relapse prevention)

Safety warning: avoid “retatrutide” sold online or on social media

Because retatrutide is not approved for UK use, any product sold as retatrutide outside authorised trials is a serious risk. The MHRA has explicitly warned such products are likely illegal and potentially dangerous, and has carried out major enforcement action against unlicensed supply.

If you’re considering weight-loss injections, only do so via a regulated prescriber and a legitimate pharmacy supply chain.

What to do now (if you’re looking for help today)

Retatrutide may be the future, but many people need support now.

At Puri Pharmacy, we focus on:

• safe, clinically led weight management

• evidence based prescribing where appropriate

• structured follow up to support tolerability, adherence and long term results

Be Ready for What’s Next (Without Taking Risks)

Retatrutide isn’t available in the UK yet, but you can get the right support now.

Book a free consultation to discuss regulated weight-loss options, side-effect support, and a plan that keeps you progressing while we track new treatments as they emerge.

Book your free consultation