The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
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Weight Loss Injections: Understanding Retatrutide
Retatrutide is a new injectable weight management medication currently in clinical trials, not yet approved by the MHRA for use in the UK. Unlike Wegovy and Mounjaro, which are already licensed for weight management in the UK, retatrutide remains experimental. This guide explains what retatrutide is, how it works, current trial data, and where it fits in the landscape of prescription weight loss medications available through NHS weight loss services and private prescribing in the UK.
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Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
What Is Retatrutide and How Does It Work?
Retatrutide is a triple hormone receptor agonist—meaning it activates three natural hormone pathways rather than one or two like existing medications. It stimulates GIP, GLP-1, and glucagon receptors, creating a more potent effect on appetite regulation, satiety, and metabolic rate. Early clinical data suggests this triple action may deliver greater average weight loss than dual-action Mounjaro (tirzepatide) or single-action Wegovy (semaglutide). However, retatrutide is still in clinical trials and is not yet approved by the MHRA for prescription use in the UK or Europe. Development and regulatory approval typically takes several years. When discussing weight management options for UK patients, GPs and prescribers currently recommend licensed medications: Mounjaro, Wegovy, or Saxenda.
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How It Works
Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
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Retatrutide Clinical Trial Data and Expected Outcomes
In the SUMMIT trials (ongoing multi-year studies), retatrutide at the highest doses showed average weight loss of approximately 24% of body weight over 68 weeks—higher than Mounjaro's average of 20.9% or Wegovy's 14.9%. However, these are early-stage trial results, and individual outcomes vary significantly. Importantly, trial participants combined medication with reduced-calorie diets and increased physical activity. Retatrutide also showed a higher rate of gastrointestinal side effects compared to dual-action medications. Full safety and efficacy data are still being collected, which is why regulatory approval has not yet occurred in the UK or US.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
Is retatrutide approved in the UK right now?
No. Retatrutide is not yet approved by the MHRA. It remains in clinical trials. Currently, the only licensed weight loss injections in the UK are Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). If you need weight management support now, speak to your GP about these licensed options.
How does retatrutide differ from Mounjaro and Wegovy?
Retatrutide is a triple hormone agonist (GIP, GLP-1, glucagon), while Mounjaro is dual (GIP, GLP-1) and Wegovy is single (GLP-1 only). Early trials suggest retatrutide may produce greater average weight loss, but it's not yet approved. When comparing licensed medications, use our GLP-1 weight loss calculator to estimate potential outcomes with Mounjaro or Wegovy based on your health profile.
When will retatrutide be available in the UK?
There is no confirmed UK approval date yet. Regulatory approval typically requires 2–3 years of additional trial data after trials conclude. Once approved, it would likely be available through specialist NHS weight management services and private prescribing, subject to individual clinical assessment and eligibility.
What are the side effects of retatrutide?
Common side effects in trials include nausea, vomiting, diarrhoea, and constipation—similar to other GLP-1 medications. Retatrutide showed a higher rate of gastrointestinal effects than Mounjaro or Wegovy in early trials. Serious side effects are rare but include pancreatitis and gallbladder issues. Full safety data are still being collected.
Can I get retatrutide privately in the UK now?
No. Retatrutide is not licensed anywhere in the world yet, so it cannot be legally prescribed in the UK, whether through NHS or private services. Prescription weight loss options in the UK are limited to licensed medications. Check your eligibility using our free BMI calculator, then discuss options with your GP or a licensed prescriber.
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