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 in the UK
Weight loss injections are prescription-only medicines that help regulate appetite and support weight management in UK adults. The most commonly prescribed options are GLP-1 receptor agonists like Wegovy (semaglutide) and Mounjaro (tirzepatide), which work alongside diet and exercise. These medications are licensed for use in the UK and available through both NHS specialist services and private prescribing following a clinical assessment. Individual results vary significantly, and injections are most effective when combined with lifestyle changes. Understanding how they work, who can access them, and what to expect is essential before considering treatment.
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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 Weight Loss Injections Work
Weight loss injections available in the UK are GLP-1 receptor agonists or dual GIP/GLP-1 agonists that mimic natural hormones produced after eating. These medications signal to the brain that you're full, reduce food cravings, and slow how quickly food leaves the stomach — supporting smaller portion sizes and reduced calorie intake.
Wegovy (semaglutide) activates GLP-1 receptors and is a once-weekly injection. Mounjaro (tirzepatide) is a dual GIP/GLP-1 agonist, also once-weekly. Both are prescription-only and require a clinical assessment before being prescribed. In clinical trials, Wegovy supported an average weight loss of around 15% of body weight, while Mounjaro showed average losses of 15–21% — though results vary considerably between individuals.
These injections are not insulin and do not work like insulin. They're used alongside a reduced-calorie diet and increased physical activity for optimal results. Many people also find it helpful to track their progress using a free progress tracker to monitor weight loss over time.
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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.
UK Access and Eligibility
In the UK, weight loss injections are available through NHS specialist tier 3 weight management services or privately through GPhC-registered online prescribers. NHS access requires referral from your GP, though availability and waiting times vary significantly by region. Private prescription is faster and does not require NHS referral, though costs apply.
You may be eligible if you have a BMI of 30 or above, or a BMI of 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or sleep apnoea. Adults of South Asian, Chinese, Black African, or Black Caribbean heritage may have access at lower BMI thresholds due to higher health risks at those weights. A prescriber will assess whether injection therapy is suitable based on your individual health, other medications, and medical history.
- 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 injections safe?
Weight loss injections like Wegovy and Mounjaro are licensed and approved by the MHRA. Common side effects include nausea, diarrhoea, and constipation — usually mild and temporary. Serious side effects are rare. Always disclose all medical conditions and medications to your prescriber. If you experience severe stomach pain, allergic reactions, or vision changes, seek immediate medical attention.
What's the difference between Wegovy and Mounjaro?
Both are once-weekly injections for weight management. Wegovy (semaglutide) is a GLP-1 agonist with an average weight loss of around 15%. Mounjaro (tirzepatide) is a dual GIP/GLP-1 agonist with average losses of 15–21%. Mounjaro has a higher starting dose and different escalation schedule. Both require prescription and clinical assessment. Your prescriber will advise which suits your circumstances.
How quickly will I lose weight with injections?
Weight loss typically begins within the first 2–4 weeks and continues as your dose increases over several months. The rate varies — some people lose weight steadily, others plateau temporarily. Average losses are 0.5–1 kg per week alongside diet and exercise changes. Using a free BMI calculator helps track your progress and understand your health metrics throughout treatment.
Do weight loss injections work without diet and exercise?
No. Weight loss injections are most effective alongside a reduced-calorie diet and increased physical activity. The medication helps by reducing appetite and cravings, but you still need to eat less and move more. NHS guidance emphasises that injections are a tool to support lifestyle change, not a substitute for it. Sustainable weight loss requires behaviour change.
What happens when you stop taking weight loss injections?
Weight regain is possible after stopping injections, though this varies. Many people maintain some weight loss if they've adopted lasting dietary and exercise habits. Some regain weight over time. Your prescriber will discuss long-term management plans with you. Ongoing lifestyle changes — including regular physical activity and mindful eating — are key to preventing regain after treatment ends.
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.