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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Lose Weight Jab: How Weight Loss Injections Work in the UK
A "weight loss jab" is a prescription-only injection designed to support weight management in UK adults. The most commonly prescribed weight loss injections in the UK are semaglutide (Wegovy) and tirzepatide (Mounjaro). These medications work by mimicking natural hormones that help reduce appetite and support feeling fuller after eating. They are used alongside sustainable weight loss methods like reduced-calorie diet and increased physical activity. Access is available through NHS specialist weight management services or private online prescribing following 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 Injections Work
Weight loss jabs contain GLP-1 receptor agonist medications (or in Mounjaro's case, a dual GIP/GLP-1 agonist). These mimic natural hormones your gut produces after eating. They signal to your brain that you are satisfied, helping reduce appetite and food cravings. The medications also slow how quickly food leaves your stomach, which supports feeling fuller for longer.
Wegovy (semaglutide) is a once-weekly GLP-1 injection. Clinical trials (STEP-1) showed an average weight loss of around 15% of body weight over 68 weeks when combined with a reduced-calorie diet and physical activity. Mounjaro (tirzepatide) uses a dual mechanism and clinical data (SURMOUNT-1) showed average weight loss of 15–21% depending on dose.
Important: Individual results vary significantly. These medications are prescription-only and require assessment by a licensed prescriber to confirm suitability for you.
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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 Eligibility and How to Access
You may be eligible for a weight loss jab in the UK 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 cardiovascular disease. For people of South Asian, Chinese, Black African, or Black Caribbean heritage, lower BMI thresholds may apply.
NHS access: Referral to a tier 3 specialist weight management service through your GP. Availability varies by region and waiting times can be lengthy. Private access: Online consultation with a UK-licensed prescriber, available without NHS referral. Our GLP-1 weight loss calculator can help you understand potential outcomes at different doses. Your GP can discuss which access route suits you best.
- 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 a weight loss jab safe?
Weight loss jabs are prescription-only medicines regulated by the MHRA. They have undergone rigorous clinical trials. Common side effects include nausea, diarrhoea, and constipation—usually mild and temporary. Serious side effects are rare but can include severe stomach pain (possible pancreatitis). A licensed prescriber assesses your individual health before prescribing to ensure suitability.
How much weight will I lose with a weight loss jab?
Clinical trials show average weight loss of 15–21% of body weight over several months, depending on the medication and dose. However, individual results vary significantly—some people lose more, some less. Weight loss jabs work alongside reduced-calorie diet and physical activity, not instead of them. Use our GLP-1 weight loss calculator to see projected ranges for your situation.
Is Ozempic the same as Wegovy?
Both contain semaglutide, but they are different medicines. Ozempic is approved for type 2 diabetes (doses up to 2 mg) and is not licensed for weight loss in the UK. Wegovy is the licensed weight management version (doses up to 2.4 mg). Using Ozempic for weight loss is off-label. Always ask your prescriber which medication is appropriate for you.
How do I get a weight loss jab on the NHS?
Ask your GP for a referral to your local tier 3 specialist weight management service. They will assess your eligibility based on BMI and health conditions. NHS access varies significantly by region—some areas have shorter waiting lists than others. If NHS waiting times are long, you can also access weight loss injections privately through a UK-licensed online prescriber.
Can I use a weight loss jab long-term?
Yes, weight loss jabs can be used long-term under medical supervision. However, stopping the medication may lead to gradual weight regain. Long-term use requires ongoing GP or prescriber monitoring to ensure safety and effectiveness. Discuss with your prescriber how long treatment is appropriate for your situation and how to maintain results if you stop.
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.