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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Peptides for Weight Loss: UK Guide to GLP-1 Medications
Peptides for weight loss are prescription-only medications that work by mimicking natural hormones in your body. In the UK, the most commonly prescribed weight loss peptide is semaglutide (Wegovy), a GLP-1 receptor agonist that may help reduce appetite and support weight management alongside diet and exercise. Another option is tirzepatide (Mounjaro), which activates two hormone pathways. Both are licensed for weight management in UK adults meeting specific BMI and health criteria. Access is available through NHS specialist services or private online prescribing following a clinical assessment.
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Calculate Your BMI Instantly
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.
How Weight Loss Peptides Work in Your Body
GLP-1 peptides like semaglutide mimic glucagon-like peptide-1, a natural hormone produced in your gut after eating. This hormone signals to your brain that you feel full, helping reduce appetite and food cravings. The peptide also slows gastric emptying — how quickly food leaves your stomach — which may help you feel satisfied after smaller meals.
Tirzepatide (Mounjaro) works differently: it activates two hormone pathways (GIP and GLP-1), which is why it's called a dual agonist. This dual action may explain why some people experience greater weight loss with tirzepatide compared to GLP-1-only medications like Wegovy.
- Semaglutide (Wegovy): activates GLP-1 pathway only
- Tirzepatide (Mounjaro): activates GIP and GLP-1 pathways
- Both reduce appetite and support feeling fuller longer
- Both are once-weekly injections
Use our free GLP-1 weight loss calculator on this page to explore potential outcomes at different doses based on clinical trial data.
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Who Can Get Weight Loss Peptides in the UK?
To be eligible for prescription weight loss peptides in the UK, you typically need:
- BMI of 30 or above, OR
- BMI of 27–29.9 with at least one weight-related health condition (type 2 diabetes, high blood pressure, heart disease, or sleep apnoea)
Adults of South Asian, Chinese, Black African, or Black Caribbean heritage may qualify at slightly lower BMI thresholds (27.5+) because these groups face higher health risks at lower weights according to NHS guidance.
Access your weight status using our free BMI calculator above, then discuss prescription weight loss options with your GP or a licensed prescriber. Neither semaglutide nor tirzepatide is suitable during pregnancy or whilst breastfeeding. Individual assessment is always required.
- 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.
What's the difference between Wegovy and Ozempic?
Both contain semaglutide, but Wegovy is specifically licensed for weight management at doses up to 2.4mg. Ozempic is licensed for type 2 diabetes management only (up to 2mg). Using Ozempic for weight loss is off-label in the UK. Wegovy is the licensed weight loss version and is prescribed through weight management services or private prescribers.
How much weight can you lose on peptides?
Clinical trials showed semaglutide (Wegovy) helped participants lose approximately 14.9% of body weight on average over 68 weeks, alongside reduced-calorie diet and exercise. Tirzepatide (Mounjaro) showed average losses of 15–21% depending on dose. Individual results vary significantly — some people lose more, some less. Weight loss is not assured.
Are weight loss peptides available on the NHS?
Yes, tirzepatide (Mounjaro) and semaglutide (Wegovy) are available on the NHS through specialist tier 3 weight management services. However, availability varies significantly by region, and waiting lists can be very long. Private prescribing is available without an NHS referral through licensed online prescribers.
What are the main side effects of weight loss peptides?
Very common side effects include nausea, diarrhoea, vomiting, constipation, and stomach pain — especially when starting or increasing doses. These often improve over time. Less common side effects include headache, fatigue, dizziness, and injection site reactions. Serious side effects are rare but require immediate medical attention.
Can weight loss peptides be used without diet and exercise changes?
No — weight loss peptides work best alongside a reduced-calorie diet and increased physical activity. Clinical trials that showed weight loss results all included lifestyle changes. The peptide supports your efforts by reducing appetite, but dietary choices and exercise remain essential for sustainable results. Your prescriber will discuss this during your assessment.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.