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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No Side Effects On Mounjaro — What the Clinical Data Shows
The short answer is that both Mounjaro (tirzepatide) and Wegovy (semaglutide) commonly cause side effects during treatment, though not everyone experiences them. Clinical trials showed most people do have some gastrointestinal effects, particularly nausea and stomach discomfort. However, individual experience varies significantly — some people tolerate either medication better than others. This guide compares the side effect profiles of both prescription weight loss injections based on actual SURMOUNT and STEP trial data, and explains why one might suit you better than the other.
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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.
Clinical Trial Side Effect Data: SURMOUNT vs STEP Trials
The SURMOUNT-1 trial (Mounjaro, tirzepatide) and STEP-1 trial (Wegovy, semaglutide) measured side effects carefully. In SURMOUNT-1, the most common side effects were nausea (25% of participants), diarrhoea (22%), vomiting (9%), and constipation (21%). These side effects were often mild-to-moderate and tended to decrease over time. In STEP-1, the side effect profile was broadly similar: nausea (25–33%), diarrhoea (19–23%), vomiting (7–11%), and constipation (11–21%), depending on dose. Neither medication showed a clear advantage in terms of fewer side effects — both caused similar gastrointestinal effects. The key difference is not side effect absence, but tolerability and whether effects improved with time, which varied between individuals. Using our free BMI calculator and GLP-1 weight loss calculator can help you understand your eligibility and projected outcomes before discussing either medication with your prescriber.
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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.
Individual Suitability and Prescriber Decision
Because side effect profiles are comparable between Mounjaro and Wegovy, your prescriber will choose based on other factors: your BMI, existing health conditions, medication interactions, and personal preference. Both are prescription-only and require clinical assessment. The dual mechanism of Mounjaro (GIP plus GLP-1 agonist) versus Wegovy's single GLP-1 mechanism does not assures fewer side effects — clinical data shows tolerability depends on dose titration, individual stomach sensitivity, and diet changes. Starting at the lowest dose and escalating slowly helps most people adapt. If you experience troublesome side effects on one medication, switching to the other may help, but this requires prescriber guidance. Your GP or licensed prescriber can assess which option suits your health profile, and whether slower dose escalation might improve tolerability 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.
Does Mounjaro have fewer side effects than Wegovy?
No — clinical trial data shows comparable side effect rates. Both commonly cause nausea, diarrhoea, and constipation, particularly early in treatment. Individual tolerance varies, but neither medication offers a clear advantage in side effect reduction. Your prescriber will choose based on your specific health profile.
Can you take Mounjaro without any side effects?
Some people experience minimal or no noticeable side effects, but clinical trials show the majority do experience some gastrointestinal symptoms, at least initially. Most people find effects improve as their body adjusts over weeks or months. Starting at the lowest dose (2.5mg Mounjaro) and escalating slowly helps reduce side effect severity.
What percentage of people get side effects on weight loss injections?
In clinical trials, approximately 70–80% of people on both Mounjaro and Wegovy experienced at least one gastrointestinal side effect. However, many were mild, temporary, and resolved with time. Serious side effects were rare in both trials.
Is Mounjaro or Wegovy better tolerated?
Clinical data does not show clear superiority for either medication in terms of tolerability. Both have similar side effect profiles. Which is better tolerated depends on individual stomach sensitivity, dose escalation pace, and lifestyle factors. Your prescriber will assess which suits you based on your medical history.
Should I choose Mounjaro if I'm worried about side effects?
If side effects concern you, discuss dose titration strategies with your prescriber — starting at the lowest dose (2.5mg for Mounjaro, 0.25mg for Wegovy) and escalating slowly improves many people's tolerance. Both medications work alongside dietary changes; adjusting diet gradually also helps minimise gastrointestinal effects.
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.