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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What Is the Best Weight Loss Jab in the UK?
There is no single "best" weight loss jab — the most suitable prescription weight loss injection depends on your individual health, BMI, and medical history. In the UK, the two main licensed options are Wegovy (semaglutide) and Mounjaro (tirzepatide), both prescription-only medicines that may help support weight loss when used alongside diet and exercise. A GP or licensed prescriber will assess which, if either, is appropriate for you following a clinical evaluation. This guide explains how these medications work, what to expect, and how to access them through the NHS or private prescribing.
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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 Jabs Work — The Science
Wegovy (semaglutide) is a GLP-1 receptor agonist — a medication that mimics a natural hormone your body produces after eating. It works by signalling your brain that you are full, reducing appetite and food cravings, and slowing how quickly food leaves your stomach. In the STEP-1 clinical trial, participants taking Wegovy lost an average of approximately 14.9% of their body weight over 68 weeks, alongside a reduced-calorie diet and increased physical activity.
Mounjaro (tirzepatide) works differently — it activates two hormone pathways (GIP and GLP-1), offering a dual mechanism. Clinical trials showed participants lost an average of 15–21% of their body weight depending on dose. Both are once-weekly injections, prescription-only, and require a clinical assessment before prescribing.
Try our free GLP-1 weight loss calculator to see projected weight loss outcomes at different doses, and use our BMI calculator to check whether you may be eligible for prescription weight loss support.
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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: NHS vs Private Prescribing
NHS access: Prescription weight loss medications are available through NHS specialist tier 3/4 weight management services. You'll need a GP referral and may face significant waiting lists depending on your region. Access is restricted to adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions (type 2 diabetes, high blood pressure, cardiovascular disease, sleep apnoea).
Private access: Both Wegovy and Mounjaro are available through online prescribing services following a digital clinical assessment. This route doesn't require NHS referral and can be faster, though costs range from approximately £129–£320 per month depending on dose.
Either route requires a valid prescription from a UK-licensed prescriber — weight loss jabs cannot be purchased without clinical assessment.
- 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 a weight loss jab?
No. Ozempic contains semaglutide but is licensed for type 2 diabetes management (up to 2 mg dose), not weight loss. Wegovy is the licensed weight management version of semaglutide (2.4 mg). Using Ozempic for weight loss is off-label prescribing and not recommended. Always ask your prescriber which medication they are prescribing and why.
Which weight loss jab is most effective?
Clinical trials show both Wegovy and Mounjaro produce meaningful weight loss — typically 15–21% of body weight over several months. Mounjaro's dual mechanism may offer slightly greater average weight loss, but individual results vary significantly. Your prescriber will recommend the most suitable option based on your health profile and medical history.
How quickly will I lose weight with a weight loss jab?
Weight loss is gradual. Most people lose 0.5–1 kg per week, though this varies. Doses are increased gradually over 16–20 weeks, so initial weeks focus on tolerating the medication. Most significant weight loss occurs months 2–6. Your GP or prescriber can discuss realistic timelines for your situation.
Are weight loss jabs safe?
Both Wegovy and Mounjaro have been through clinical trials and are approved by the MHRA. However, they are not suitable for everyone — contraindications include pregnancy, certain medical conditions, and specific medications. Common side effects include nausea, diarrhoea, and constipation. Serious side effects are rare. Your prescriber will assess safety during your clinical assessment.
What happens when I stop using a weight loss jab?
Weight regain is possible when treatment stops — most people maintain 50–65% of their weight loss long-term if they sustain lifestyle changes. Weight loss jabs are designed to support behaviour change (diet and exercise), not replace it. Your prescriber will discuss long-term management options with you.
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.