Retatrutide is an investigational once-weekly injection being developed by Eli Lilly that targets three hormone receptors involved in appetite and metabolism: GIP, GLP-1, and glucagon. Because it activates three pathways (rather than one or two), it’s often described as a “triple agonist” and has attracted attention for the level of weight loss seen in clinical trials.
This guide explains what retatrutide is, how it works, what trial results show so far, common side effects reported in trials, and its legal status in the UK.
What is retatrutide?
Retatrutide is a single molecule designed to activate three receptors:
- GLP-1 (glucagon-like peptide-1) receptor
- GIP (glucose-dependent insulinotropic polypeptide) receptor
- Glucagon receptor
In trials, it is administered as a once-weekly subcutaneous injection. It is not yet approved for routine prescribing and remains an investigational medicine.
Why it’s called a “triple agonist”
Most people are familiar with GLP-1 medicines because of drugs such as semaglutide (Wegovy/Ozempic). Mounjaro (tirzepatide) is a dual agonist (GLP-1 + GIP). Retatrutide adds a third target: glucagon.
Conceptually, the triple mechanism aims to combine:
- Appetite reduction and improved satiety (strongly associated with GLP-1 effects)
- Additional incretin and metabolic effects (GIP)
- Increased energy expenditure and metabolic signalling (glucagon pathway), though glucagon activity is also why researchers watch tolerability and cardiometabolic markers closely
What do studies show so far?
Phase 2 results (obesity/overweight, no diabetes)
Lilly reported phase 2 results presented at ADA 2023 and published in the New England Journal of Medicine, including:
- Up to 17.5% mean weight reduction at 24 weeks (highest dose group, per Lilly’s summary)
- A secondary endpoint showing up to 24.2% mean weight reduction at 48 weeks (per Lilly’s summary)
These were clinical trial findings in a controlled setting with structured dosing and follow-up.
Phase 3 results (TRIUMPH-4)
In December 2025, Lilly announced positive topline results from TRIUMPH-4 (obesity/overweight with knee osteoarthritis, without diabetes). Reported outcomes at 68 weeks included:
- Mean weight loss of 26.4% (9 mg) and 28.7% (12 mg) vs 2.1% placebo
- Improvements in knee osteoarthritis pain and physical function (WOMAC measures)
- A high proportion of participants reaching ≥25% weight loss (notable for obesity drug trials)
These are topline results; full peer-reviewed publication details and broader phase 3 readouts are expected over 2026.
Side effects and tolerability
In the available reporting, the most common side effects look similar to other incretin-based medicines, especially gastrointestinal effects, such as:
- Nausea
- Diarrhoea
- Constipation
- Vomiting
- Decreased appetite
Some reporting from the TRIUMPH-4 topline announcement also noted dysesthesia in a proportion of participants at higher doses (reported as generally mild in that coverage). The exact rates, dose relationships, and discontinuation patterns are best interpreted from full trial publications and regulatory reviews once available.
Is retatrutide available in the UK?
No. Retatrutide is not available for public prescribing. Lilly states it is investigational and still being studied in phase 3 clinical trials across multiple conditions, and availability depends on completion of trials and regulatory approval processes.
Legal status in the UK and “grey market” risks
Because retatrutide is not approved for routine use, any “retatrutide” being sold online to consumers is a major red flag.
Investigations and reporting have highlighted the sale of unlicensed “retatrutide” through online sellers and the risks of counterfeit or unregulated supply chains. These products may be falsely labelled, incorrectly dosed, contaminated, or not the drug they claim to be.
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How retatrutide compares to currently available options
Retatrutide is being studied as a next-generation medicine, but it is not yet approved or prescribable. By contrast, established prescription options (for eligible patients) are supplied via regulated pathways with:
- Confirmed sourcing and quality controls
- Clinical assessment and appropriate dosing decisions
- Ongoing monitoring and safety advice
That difference (regulated prescribing vs unlicensed/unknown supply) is the biggest safety separator right now.
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At PPRX UK, we provide:
- Genuine Mounjaro pens, sourced directly from the manufacturer (Eli Lilly)
- Dispensing via a GPhC-registered UK pharmacy
- Professional consultations with UK pharmacists
- Ongoing support, dosage guidance, and repeat supply options
Whether you’re just starting or switching from Ozempic or Wegovy, we’re here to help.
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Mounjaro is a prescription-only medicine. This article is for informational purposes only and does not replace medical advice. Always consult a qualified healthcare provider before starting treatment. nhs
