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
Free tracker included with every download this week
What Are Weight Loss Peptides?
Weight loss peptides are prescription-only injectable medications that mimic natural hormones to help regulate appetite and support weight management. In the UK, the most commonly prescribed weight loss peptides are semaglutide (Wegovy) and tirzepatide (Mounjaro). These are licensed for adults with a BMI of 30 or above, or 27 or above with weight-related health conditions such as type 2 diabetes or high blood pressure. They work alongside a reduced-calorie diet and increased physical activity. Use our free GLP-1 weight loss calculator above to explore potential outcomes, and check your BMI using our free BMI calculator to understand your eligibility.
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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
Weight loss peptides are GLP-1 receptor agonists — they activate a natural hormone pathway in your body. GLP-1 is a hormone produced in the gut after eating that signals fullness to the brain. Prescription weight loss peptides amplify this effect, which may help reduce appetite, decrease food cravings, and support feeling fuller after smaller meals.
- Semaglutide (Wegovy): A once-weekly GLP-1 peptide. Clinical trials (STEP programme) showed average weight loss of around 15% of body weight over 68 weeks alongside lifestyle changes.
- Tirzepatide (Mounjaro): A dual GIP and GLP-1 peptide — activates two hormone pathways. SURMOUNT trial data showed average weight loss of 15–21% depending on dose.
Both are prescription-only and require clinical assessment. Results vary significantly between individuals. Weight loss peptides are not insulin and do not work in the same way as diabetes medications, though they may help blood sugar control.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
UK Eligibility and How to Access Weight Loss Peptides
In the UK, you may be eligible for prescription weight loss peptides if you have a BMI of 30 or above, or BMI of 27–29.9 with a weight-related health condition such as type 2 diabetes, prediabetes, high blood pressure, or cardiovascular disease. Adults of South Asian, Chinese, Black African, or Black Caribbean heritage may be eligible at lower BMI thresholds (27.5+) due to higher health risks at lower weights.
NHS access: Referral to a tier 3 specialist weight management service through your GP. Private access: Online consultation with a UK-licensed prescriber followed by prescription from a GPhC-registered pharmacy. NHS availability varies significantly by region and waiting times can be substantial. Use our free progress tracker to monitor your weight management journey alongside any treatment.
- 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.
Are weight loss peptides the same as slimming injections?
Yes — "slimming injections" and "weight loss injections" are colloquial UK terms for GLP-1 and dual GIP/GLP-1 receptor agonist medications like Wegovy and Mounjaro. They are prescription-only injectable medicines, not cosmetic injections. They require clinical assessment and are used alongside diet and exercise.
What's the difference between Wegovy and Ozempic?
Both contain semaglutide, but Wegovy (2.4 mg) is licensed in the UK specifically for weight management. Ozempic (up to 2 mg) is licensed for type 2 diabetes. Using Ozempic for weight loss is off-label. Wegovy is the licensed weight loss version and is prescribed within appropriate weight management programmes.
How quickly will I lose weight on weight loss peptides?
Results vary significantly. Clinical trials showed average weight loss of 15–21% over 68–72 weeks alongside a reduced-calorie diet and increased physical activity. The NHS recommends sustainable loss of 0.5–1 kg per week. Some people lose weight faster initially, others more gradually. Your prescriber can discuss expected timelines.
What are the common side effects of weight loss peptides?
Very common side effects include nausea, diarrhoea, constipation, stomach pain, and indigestion — especially when starting or increasing doses. These often improve over time. Serious side effects are rare but can include severe stomach pain (possible pancreatitis) or gallbladder problems. Always report new symptoms to your prescriber.
Can I get weight loss peptides on the NHS?
Yes — through specialist tier 3 weight management services following a GP referral. However, NHS availability and waiting times vary significantly across different regions in the UK. Private prescription routes are also available through online consultations with UK-licensed prescribers, though these involve private costs.
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.