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 Is Mounjaro and How Does It Work?
Mounjaro (tirzepatide) is a prescription-only weight loss injection licensed in the UK for adults with obesity or overweight with weight-related health conditions. It works differently from other weight loss medications because it's a dual GIP and GLP-1 receptor agonist — meaning it activates two natural hormone pathways to help regulate appetite, reduce food cravings, and support feeling fuller after meals. You inject it once weekly under the skin. To understand how your weight compares to health guidelines, try our free BMI calculator above to check your personal BMI instantly.
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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.
Mounjaro vs Wegovy: Key Differences and Clinical Evidence
Both Mounjaro and Wegovy are prescription-only weekly injections for weight management in the UK, but they work through different mechanisms. Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy contains semaglutide, a GLP-1 receptor agonist only. This difference in mechanism may explain their different weight loss results in clinical trials.
- Mounjaro (SURMOUNT trial): Participants on the maximum 15mg dose lost an average of 20.9% of their body weight over 72 weeks alongside diet and exercise changes.
- Wegovy (STEP-1 trial): Participants on 2.4mg lost an average of 14.9% of body weight over 68 weeks with the same lifestyle support.
Individual results vary significantly based on genetics, adherence, diet, and physical activity. Both require a clinical assessment before prescription, and both are most effective when combined with reduced-calorie eating and increased physical activity.
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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.
How to Access Mounjaro in the UK and Which Is Right for You
In the UK, Mounjaro is available through NHS specialist tier 3 weight management services (though availability and waiting times vary by region) or through private online prescribing following a clinical assessment. Your prescriber will assess your individual health, BMI, any weight-related health conditions, and current medications to determine suitability. Neither medication is "better" — the right choice depends on your personal circumstances. Use our GLP-1 weight loss calculator to explore potential weight loss projections at different doses, and our weight converter tool to track your progress in stone, lbs, or kg throughout your journey.
- 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 Mounjaro the same as Ozempic or Wegovy?
No. Mounjaro contains tirzepatide (a dual GIP and GLP-1 agonist), while Wegovy contains semaglutide (GLP-1 only). Ozempic also contains semaglutide but is licensed for type 2 diabetes, not weight loss. All three are different medications with different mechanisms and dosing schedules.
How much weight can I expect to lose on Mounjaro?
Clinical trials showed an average weight loss of 15–21% of body weight depending on dose, alongside diet and exercise changes. However, individual results vary significantly. Your prescriber can discuss realistic expectations based on your personal health. Weight loss is not assured for every person.
Do I need a GP referral to get Mounjaro in the UK?
For NHS access, you typically need a referral to a specialist weight management service, which involves a GP discussion. For private prescribing, you can access online consultations directly without NHS referral. Both routes require a clinical assessment from a licensed UK prescriber.
What are the main side effects of Mounjaro?
Very common side effects include nausea, diarrhoea, vomiting, and constipation, especially when starting or increasing doses. These often improve over time. Serious side effects are rare but include severe stomach pain (possible pancreatitis) and gallbladder problems. Always discuss side effect concerns with your prescriber.
Can I switch from Wegovy to Mounjaro?
Switching between medications should only be done under prescriber guidance. There is no official conversion chart, but approximate clinical equivalences exist. Your prescriber will assess whether a switch is appropriate and manage the transition safely. Never change medications without professional advice. ```
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.