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 2.5mg vs Wegovy: What's the Difference?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections licensed in the UK, but they work in different ways and at different starting doses. Mounjaro starts at 2.5mg once weekly, while Wegovy begins at 0.25mg. Understanding how these medications differ — in mechanism, dosing, side effects, and clinical trial results — can help you and your GP make an informed decision about which might be suitable for your circumstances. Both require a clinical assessment before prescription.
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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 is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways in the gut. GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) together help regulate appetite, reduce cravings, and support feeling fuller after smaller meals.
Wegovy is a GLP-1 receptor agonist only, activating just the GLP-1 pathway. This single mechanism has also been shown to reduce appetite and support weight loss, though through one signalling route rather than two.
In clinical trials, the dual mechanism of Mounjaro showed slightly higher average weight loss — approximately 20.9% of body weight at the maximum 15mg dose (SURMOUNT-1 trial) compared to Wegovy's approximately 14.9% at 2.4mg (STEP-1 trial). However, individual results vary significantly, and both medications work best alongside a reduced-calorie diet and increased physical activity.
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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.
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Dosing and Titration: Why Mounjaro Starts Higher
Mounjaro's starting dose is 2.5mg once weekly — four times higher than Wegovy's 0.25mg starting dose. This does not mean Mounjaro is stronger at baseline; it reflects the different potency of the two mechanisms. Mounjaro typically escalates in 2.5mg increments every 4 weeks (2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg), while Wegovy escalates more gradually in smaller steps (0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg). Both follow a KwikPen injection system and are dosed once weekly. Your prescriber will recommend the schedule most suitable for you based on your individual response and tolerance.
Both medications require clinical assessment to confirm you meet NHS eligibility criteria: a BMI of 30kg/m² or above, or 27–29.9kg/m² with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease. Your choice between the two is not yours alone to make — your prescriber will assess which is more appropriate based on your medical history, current medications, and individual health profile. Some people may be better suited to one over the other depending on how they tolerate side effects, whether they have type 2 diabetes, or other personal factors. Both are available through NHS specialist weight management services (with regional variation) and through private online assessment and prescription.
- 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 2.5mg stronger than Wegovy 0.25mg?
No — they are not directly comparable at these doses. Mounjaro 2.5mg is the starting dose and includes dual GIP+GLP-1 action. Wegovy 0.25mg is a quarter of Mounjaro's starting dose and activates GLP-1 only. Different mechanisms and potencies mean the doses cannot be simply ranked as "stronger" or "weaker". Your prescriber will discuss which suits you.
Which caused more weight loss in clinical trials?
Mounjaro (SURMOUNT-1) showed average weight loss of approximately 20.9% at maximum dose, versus Wegovy (STEP-1) at approximately 14.9%. However, these are trial averages — individual results vary considerably. Some people lose more on Wegovy, some on Mounjaro.
What are the main side effects of Mounjaro 2.5mg?
Common side effects at the starting dose include nausea, diarrhoea, vomiting, constipation, and stomach pain. These often ease after a few weeks as your body adjusts. Serious side effects are rare but include severe stomach pain (possible pancreatitis) — seek urgent medical attention if this occurs.
Can I switch between Mounjaro and Wegovy?
Switching is possible but must be managed by your prescriber. There is no exact dose equivalence between the two medications, so your prescriber will recommend an appropriate Wegovy dose based on your Mounjaro dose and clinical response. Never switch without medical guidance.
How do I access Mounjaro or Wegovy on the NHS in the UK?
Both require referral to a specialist tier 3 weight management service through your GP. NHS access varies significantly by region and ICB — waiting lists can be lengthy. Private online assessment is also available; a UK-licensed prescriber will assess your eligibility and issue a prescription if appropriate.
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.