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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How To Use Mounjaro Injections — Compared With Wegovy
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections used once weekly in the UK. While they work differently and have different dosing schedules, both require a clinical assessment before prescription and are administered using a simple pre-filled pen device. Understanding how each works can help you discuss options with your GP or prescriber. Our GLP-1 weight loss calculator can help you explore potential outcomes at different doses.
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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 Mounjaro and Wegovy Work Differently
The key difference lies in their mechanisms. Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways simultaneously — this dual action may contribute to its weight loss results in clinical trials. Wegovy is a GLP-1 receptor agonist only, activating a single pathway.
In the SURMOUNT clinical trial, Mounjaro users lost an average of 20.9% body weight at the maximum 15 mg dose over 72 weeks. In the STEP-1 trial, Wegovy users lost approximately 14.9% body weight at the maximum 2.4 mg dose over 68 weeks. Both studies included participants following a reduced-calorie diet and increased physical activity — results vary significantly between individuals.
Both injections slow gastric emptying and help reduce appetite. Mounjaro typically requires dose escalation over 20 weeks, whilst Wegovy reaches maximum dose in around 16 weeks. Your prescriber will determine which medication suits your individual circumstances.
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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.
Mounjaro KwikPen vs Wegovy Pen — Practical Use
Mounjaro uses a KwikPen device containing four weeks of doses (one pen = four weekly injections). Wegovy uses an auto-injection pen with a similar weekly schedule. Both are injected subcutaneously into the stomach, thigh, or upper arm, and injection sites should be rotated each week.
Mounjaro doses range from 2.5 mg to 15 mg, increasing every four weeks if tolerated. Wegovy doses range from 0.25 mg to 2.4 mg, increasing every 3–4 weeks. Both require a UK prescription following clinical assessment. Your GP or licensed prescriber will assess your BMI, health conditions, and suitability. Neither medication is currently routinely available on the NHS in most areas — access is typically through NHS specialist weight management services (where waiting lists can be lengthy) or private online prescribing following clinical consultation.
- 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.
What's the main difference between Mounjaro and Wegovy?
Mounjaro activates two hormone pathways (GIP and GLP-1), whilst Wegovy activates one (GLP-1 only). This dual mechanism is why Mounjaro may produce different results. Both are once-weekly injections, but Mounjaro uses a KwikPen and escalates to higher maximum doses. Individual results vary, and your prescriber will assess which suits you.
How do I inject Mounjaro or Wegovy?
Both come as pre-filled pens for simple subcutaneous injection. Mounjaro uses a KwikPen; Wegovy uses an auto-injection pen. You inject into your stomach, thigh, or upper arm — rotate sites each week. Your prescriber or pharmacist will provide full injection training before you start.
Which weight loss is more likely with each medication?
In clinical trials, Mounjaro at maximum dose produced average weight loss of around 20.9%, whilst Wegovy achieved approximately 14.9%. However, individual results vary significantly. Both work best alongside reduced-calorie diet and increased physical activity. Use our GLP-1 weight loss calculator to explore potential outcomes.
Can I switch from Mounjaro to Wegovy or vice versa?
Switching between these medications is possible but must be managed by your prescriber. There's no official conversion chart — approximate equivalences exist based on clinical practice, but any switch requires personalised assessment. Never stop or change medications without guidance.
How do I access Mounjaro or Wegovy on the NHS in the UK?
Both are available through NHS specialist tier 3 weight management services via GP referral, but availability and waiting lists vary significantly by region. Private access is available through UK-licensed online prescribers following a clinical consultation. Your GP can advise on local NHS access or discuss private options.
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.