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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Mounjaro vs Wegovy — Which Weekly Weight Loss Injection?
Both Mounjaro and Wegovy are prescription-only weekly weight loss injections available in the UK, but they work differently and have different starting doses. Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy is a GLP-1 agonist only. Clinical trials showed both can support weight management alongside diet and exercise, though individual results vary significantly. This guide compares the two injections to help you understand the key differences when discussing options with your prescriber. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses for either medication.
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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 Mounjaro and Wegovy Work Differently
Mounjaro (tirzepatide) activates two natural hormone pathways — GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual mechanism may help regulate appetite through two separate signals to the brain that you are full. Wegovy (semaglutide) activates GLP-1 only, using a single pathway to reduce hunger and food cravings.
In the SURMOUNT-1 clinical trial, Mounjaro at 15 mg (the maximum dose) showed an average weight loss of approximately 20.9% of body weight over 72 weeks. In the STEP-1 trial, Wegovy at 2.4 mg (its maximum dose) showed an average weight loss of approximately 14.9% of body weight over 68 weeks. Both trials included reduced-calorie diets and increased physical activity. The dual mechanism of Mounjaro may explain slightly higher average weight loss in trials, but individual responses vary considerably.
- Mounjaro: starts at 2.5 mg, escalates to maximum 15 mg
- Wegovy: starts at 0.25 mg, escalates to maximum 2.4 mg
- Both: once-weekly injections delivered via pre-filled pens
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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
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Accessing Mounjaro or Wegovy — Your Prescriber Decides
Both Mounjaro and Wegovy are prescription-only weight loss injections in the UK. Eligibility is assessed individually — your GP or a licensed prescriber will review your health history, BMI, and any weight-related conditions to determine which (if either) is appropriate for you. Use the BMI calculator on this page to check your current BMI against NHS guidelines. Some patients may find one more suitable than the other based on tolerability, cost, or prescriber preference. Neither injection is assured to work for everyone; some people respond well to one while others prefer the other. Your prescriber will discuss which weekly weight loss injection fits your circumstances.
NHS access: Available through specialist tier 3 weight management services following GP referral. Private access: Available through GPhC-registered online pharmacies following clinical assessment. Individual circumstances and regional availability will affect which option is right for you.
- 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 is the main difference between Mounjaro and Wegovy for weight loss?
Mounjaro is a dual GIP and GLP-1 receptor agonist (activates two hormone pathways), while Wegovy is a GLP-1 agonist only (activates one pathway). This dual mechanism may explain why Mounjaro showed slightly higher average weight loss in clinical trials, but individual results vary significantly.
Which weekly weight loss injection causes fewer side effects?
Both Mounjaro and Wegovy commonly cause nausea, diarrhoea, and constipation, particularly in the first few weeks. Side effect profiles are similar overall. Your experience will be individual. Most side effects improve as your body adjusts. Always discuss tolerability concerns with your prescriber.
Can I switch from Wegovy to Mounjaro?
Some patients do switch between the two injections, but this must be managed by your prescriber. There is no official conversion chart — your prescriber will assess whether a switch is appropriate and manage dosing carefully. Never switch without clinical guidance.
How quickly will I see weight loss with Mounjaro or Wegovy?
Weight loss typically begins within the first 4 weeks for many people, but this varies considerably. Clinical trials measured weight loss over many weeks and months (68–72 weeks). Sustainable weight loss is gradual. Expect slower, steadier results rather than rapid changes. Your prescriber will monitor progress.
Do I need to diet and exercise if I take Mounjaro or Wegovy?
Yes. All clinical trial results showing weight loss included a reduced-calorie diet and increased physical activity. These injections support weight management but are not a substitute for lifestyle changes. Most effective results come from combining the injection with diet and exercise modifications.
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.