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
How do weight loss injections work?
Weight loss injections available in the UK are prescription-only medications that help reduce appetite and support weight management. The most common are GLP-1 receptor agonists like Wegovy (semaglutide) and Mounjaro (tirzepatide), which work by mimicking natural hormones your body produces when eating. These injections are prescribed as part of a comprehensive weight loss uk approach that combines medication with diet, physical activity, and lifestyle support. They're not a structured approach — they're a tool to help you lose weight safely and sustain results over time.
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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.
How GLP-1 weight loss injections work
GLP-1 receptor agonists activate a natural hormone pathway in your gut that signals fullness to your brain. This helps you feel satisfied with smaller portions and reduces cravings. The medication also slows how quickly food leaves your stomach, so you feel fuller for longer. Wegovy (semaglutide) and Mounjaro (tirzepatide) are the two main prescription weight loss injections licensed in the UK. Clinical trials showed that at the maximum dose, participants lost an average of 14–21% of their body weight — though individual results vary significantly. Both are once-weekly injections. These medications work best when combined with a reduced-calorie diet and increased physical activity. They're not suitable for everyone — your GP or a licensed prescriber will assess whether you're eligible based on your BMI, health conditions, and medical history.
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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.
Eligibility and NHS access for weight loss injections
To access prescription weight loss injections in the UK, you typically need a BMI of 30 or above, or 27–29.9 if you have weight-related health conditions like type 2 diabetes, high blood pressure, or sleep apnoea. NHS weight management services can prescribe these medications, though availability varies significantly by region and waiting lists can be long. Many people access them through private online prescribing following a clinical assessment. You'll need a prescription from a UK-licensed healthcare professional — these are not available over the counter. Your prescriber will check your suitability based on your individual health profile, medications, and weight loss goals. Use our free GLP-1 weight loss calculator to understand how these injections may support your goals alongside other lifestyle changes.
- 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 Ozempic the same as Wegovy for weight loss?
Both contain semaglutide, but Ozempic is licensed for type 2 diabetes (not weight loss in the UK), while Wegovy is the licensed weight management version. Using Ozempic for weight loss is off-label prescribing. Always use the medication licensed and prescribed for your specific condition.
What are the main side effects of weight loss injections?
Common side effects include nausea, diarrhoea, constipation, and stomach discomfort — usually mild and temporary. Serious side effects are rare but include severe stomach pain (possible pancreatitis) or allergic reactions. Your prescriber will discuss side effects and monitor your response during treatment.
How much weight can I expect to lose with an injection?
Results vary significantly between individuals. Clinical trials showed average weight loss of 14–21% of body weight over several months when combined with diet and exercise. Some people lose more, some less. Your prescriber can discuss realistic expectations based on your individual circumstances.
Can I access weight loss injections through the NHS quickly?
NHS access is through tier 3 specialist weight management services, which often have waiting lists ranging from weeks to over a year depending on your region. Private online prescribing is faster but involves costs. Your GP can discuss both options with you.
Do I need to change my diet and exercise if I take a weight loss injection?
Yes. These injections work best alongside a reduced-calorie diet and increased physical activity. They're not a replacement for lifestyle changes — they're a support tool to help you sustain those changes. Sustainable weight loss requires both medication and behaviour change.
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.