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 Weight Do You Have To Be For Mounjaro and Wegovy?
Both Mounjaro and Wegovy are prescription-only weight loss injections available in the UK, and both use the same weight-based eligibility criteria. You'll typically need a BMI of 30 or above, or 27 to 29.9 if you have weight-related health conditions like type 2 diabetes or high blood pressure. However, the two medications differ in their mechanism of action and clinical trial results. This guide compares the weight requirements, how each works, and what the clinical evidence shows so you can understand what makes each suitable. Use our free BMI calculator to check where you currently stand before reading further.
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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.
Weight Requirements and Clinical Mechanisms Explained
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) follow the same NHS eligibility criteria for weight. You may be considered for either medication if you have a BMI of 30 kg/m² or above, or a BMI of 27–29.9 kg/m² alongside at least one weight-related health condition (type 2 diabetes, prediabetes, high blood pressure, dyslipidaemia, sleep apnoea, or cardiovascular disease history).
The key difference lies in how they work. Mounjaro is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways simultaneously to regulate appetite and blood sugar. Wegovy is a GLP-1 receptor agonist only — it targets a single pathway. In the SURMOUNT clinical trials, Mounjaro achieved average weight loss of approximately 15–21% of body weight, while in the STEP trials, Wegovy achieved approximately 15% average weight loss. Both require a reduced-calorie diet and increased physical activity to maximise results — the injection alone isn't enough.
Lower BMI thresholds apply if you're of South Asian, Chinese, Black African, or Black Caribbean heritage. For these groups, the NHS recommends considering treatment at BMI 27.5 or above due to higher cardiovascular risk at lower weights. A weekly weight loss injection like Mounjaro or Wegovy is typically prescribed through a UK prescription following clinical assessment — not available without a prescriber's evaluation.
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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.
Which Medication Might Suit You: Prescriber Assessment
Choosing between Mounjaro and Wegovy depends on your individual health profile, not just your weight. Both are prescription-only medicines requiring a clinical assessment by a GP or licensed prescriber. Your prescriber will review your medical history, current medications (especially oral contraceptives — both affect their absorption), kidney and liver function, and whether you have any contraindications like thyroid conditions or pancreatitis history.
Dose titration differs between the two: Mounjaro starts at 2.5 mg and escalates to a maximum of 15 mg over weeks, while Wegovy starts at 0.25 mg and reaches a maximum of 2.4 mg. Mounjaro may produce faster early weight loss due to its dual mechanism, but individual results vary significantly. You can try our free GLP-1 weight loss calculator to see projected outcomes at different doses — though remember these are clinical averages, not assures. Your prescriber will determine which is most suitable based on your specific circumstances, not on weight alone.
- 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.
Can I get Mounjaro or Wegovy on the NHS if I meet the weight requirement?
Meeting the weight requirement is necessary but not automatic. You need a BMI of 30 or above (or 27–29.9 with weight-related conditions). NHS prescribing is available through tier 3 specialist weight management services following referral from your GP. Availability varies significantly by region and waiting lists can be lengthy. Private prescribing is faster but requires payment.
What's the difference between Mounjaro and Wegovy mechanisms?
Mounjaro is a dual GIP and GLP-1 agonist — it activates two appetite and blood sugar pathways. Wegovy is a GLP-1 agonist only — single pathway activation. In clinical trials, Mounjaro showed slightly higher average weight loss (15–21% vs 15%), though individual results vary. Both require lifestyle changes alongside the injection.
Do I need to be a certain weight to start at the lowest dose?
No. Both medications are suitable for adults at the qualifying BMI threshold, regardless of starting dose. You'll begin at the lowest dose (Mounjaro 2.5 mg or Wegovy 0.25 mg) and escalate gradually over 4+ weeks. Your prescriber determines your schedule based on tolerability and response.
What if I don't qualify by BMI but have health conditions?
If your BMI is 27–29.9 and you have weight-related conditions (diabetes, high blood pressure, sleep apnoea, cardiovascular disease), you may still qualify. If you're of South Asian, Chinese, Black African, or Black Caribbean heritage, lower thresholds may apply. Always discuss with your GP.
Are both injections available as a weekly weight loss injection in the UK?
Yes. Both Mounjaro and Wegovy are once-weekly injections licensed for weight management in the UK. Liraglutide (Saxenda) is also licensed but requires daily injections. Your prescriber will discuss which suits you best based on your preference and medical history.
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.