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: Side Effects Compared
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription weight management injections available in the UK, but they work slightly differently and can cause different side effects. Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy activates only the GLP-1 pathway. This guide compares the side effects of both medications to help you understand what to expect, based on NHS guidance and clinical trial data. Always discuss which option suits you best with your GP or prescriber — individual responses vary significantly.
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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.
Clinical Trial Side Effects: SURMOUNT vs STEP Data
Side effect profiles for both medications come from large clinical trials. Mounjaro's SURMOUNT-1 trial and Wegovy's STEP-1 trial revealed similar common side effects, though frequencies and severity can differ between individuals. Both injections frequently cause nausea, diarrhoea, vomiting, and constipation — particularly during dose escalation phases. Mounjaro dose titration occurs over approximately 20 weeks (starting at 2.5 mg, escalating to 15 mg), whilst Wegovy's escalation is faster (reaching 2.4 mg by week 17). The longer Mounjaro titration schedule may mean side effects are experienced over a longer period but potentially at lower intensity during early doses. Gastrointestinal side effects typically reduce as your body adjusts to each dose, though some people continue to experience mild symptoms. Using our GLP-1 weight loss calculator, you can explore different doses and discuss realistic timelines with your prescriber.
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How It Works
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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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Key Side Effect Differences and Individual Suitability
Mounjaro's dual mechanism (GIP + GLP-1) may result in slightly different side effect patterns compared to Wegovy's single GLP-1 pathway. Some people report stronger appetite suppression with Mounjaro, which can be beneficial but occasionally leads to difficulty eating enough or fatigue if not managed carefully. Nausea appears equally common in both, but individual tolerance varies widely — what causes severe nausea in one person may be mild in another. Both medications require a clinical assessment and prescription from a UK-licensed prescriber, who will help determine which is more suitable based on your medical history, previous medication responses, and health conditions. Neither medication is "better" — suitability depends entirely on individual factors. Your prescriber considers your NHS eligibility, any weight-related health conditions, and whether you've previously used GLP-1 medications.
- 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 are the most common side effects of Mounjaro and Wegovy?
Both medications commonly cause nausea, diarrhoea, vomiting, and constipation, particularly during dose escalation. These typically reduce within days to weeks as your body adjusts. Less common side effects include fatigue, dizziness, headache, indigestion, and injection site reactions. Serious side effects are rare but require immediate medical attention — seek urgent care if you experience severe stomach pain, gallbladder problems, or serious allergic reactions.
Does Mounjaro cause worse side effects than Wegovy?
Clinical trials show similar overall side effect rates between the two medications. Individual experiences vary significantly — some people tolerate Mounjaro better, others tolerate Wegovy better. Mounjaro's longer titration schedule may spread side effects over a longer timeframe, which some people find easier to manage. Your prescriber will assess which is most suitable for you based on your individual health profile and previous medication responses.
How long do Mounjaro and Wegovy side effects last?
Most side effects are temporary and reduce within days to weeks as your body adjusts to each dose. Gastrointestinal symptoms (nausea, diarrhoea) typically improve after 3–7 days at a stable dose. If side effects persist or worsen after two weeks, contact your prescriber — dose adjustment or medication change may be needed. Some people experience mild, ongoing symptoms that they manage successfully.
Can I switch from Mounjaro to Wegovy if I get side effects?
Yes, switching between medications is possible if one causes problematic side effects. This requires discussion with your prescriber, who will manage the transition carefully. There's no official conversion chart between the two, so your prescriber will recommend an appropriate starting dose for the new medication. Switching takes time — it's not an immediate switch but a planned transition.
Which medication is prescribed more often on the NHS?
Availability varies significantly by region. Both Mounjaro and Wegovy are NICE-approved for NHS specialist weight management services, but access depends on your local NHS ICB (Integrated Care Board). Some regions have shorter waiting lists for one medication over the other. Private prescribing offers faster access to both options. Your GP can advise on NHS availability in your area or discuss private options.
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