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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Main Side Effects of Mounjaro vs Wegovy
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections available in the UK, and both commonly cause similar gastrointestinal side effects. The main difference lies in their mechanism: Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy is a GLP-1 agonist only. Understanding how their side effects compare can help you and your prescriber decide which might be more suitable following a clinical assessment. This guide compares real trial data from both medications to help you make an informed decision about weight management options.
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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 Data: SURMOUNT vs STEP
Mounjaro (tirzepatide) clinical efficacy comes from the SURMOUNT-1 trial, which showed average weight loss of approximately 20.9% body weight at the maximum 15 mg dose over 72 weeks. Wegovy (semaglutide) efficacy data comes from the STEP-1 trial, which showed average weight loss of approximately 14.9% body weight at the maximum 2.4 mg dose over 68 weeks. Both trials included participants on reduced-calorie diets and increased physical activity — individual results vary significantly. The key mechanistic difference is that Mounjaro's dual GIP+GLP-1 mechanism may provide additional appetite regulation compared to GLP-1 alone, though both medications work by slowing gastric emptying and increasing satiety. In the UK, both are available through NHS specialist weight management services or private prescribing following clinical assessment with a UK-licensed 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.
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Side Effects: What the Evidence Shows
Very common side effects affect more than 1 in 10 people on both medications and include nausea, diarrhoea, vomiting, and constipation — gastrointestinal effects are the primary concern with both drugs. Common side effects (affecting up to 1 in 10) include indigestion, fatigue, dizziness, and injection site reactions. Hair loss has been reported with both but is relatively uncommon. Serious side effects are rare but require immediate medical attention: severe abdominal pain (possible pancreatitis), gallbladder problems, and serious allergic reactions. Side effects typically improve as the body adjusts, though nausea and digestive changes can persist. Your GP or prescriber will monitor you and can adjust your dose or switch medications if side effects are intolerable. Use our free GLP-1 weight loss calculator to explore projected outcomes with either medication.
- 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?
Nausea, diarrhoea, vomiting, and constipation are very common with Mounjaro — affecting more than 1 in 10 people. These typically occur during the first few weeks and often improve over time. Less common side effects include indigestion, fatigue, dizziness, and injection site reactions. Serious side effects like severe abdominal pain are rare but require immediate medical attention.
Are Mounjaro and Wegovy side effects the same?
Both medications cause similar gastrointestinal side effects because they work on appetite hormones. The main difference is mechanism: Mounjaro uses dual GIP+GLP-1 activation, whilst Wegovy uses GLP-1 only. Individual tolerance varies, and side effects differ between people. Your prescriber can discuss which may suit you better following a clinical assessment.
How long do Mounjaro side effects last?
Most side effects are worst during the first 1–4 weeks after starting or increasing your dose. For many people, nausea and digestive symptoms improve within 2–3 weeks as the body adjusts. Some people experience persistent mild nausea or constipation throughout treatment. If side effects are severe or don't improve, contact your prescriber — dose adjustment or medication switch may help.
Can I switch from Mounjaro to Wegovy if I get side effects?
Yes, switching is possible if one medication causes intolerable side effects. There is no official conversion chart, but approximate equivalence exists (e.g. Mounjaro 10 mg is roughly equivalent to Wegovy 1.7 mg). Any switch must be managed by your prescriber to ensure safe transition and appropriate dose adjustment for your health needs.
Are Mounjaro and Wegovy safe for long-term use?
Both are licensed for long-term weight management in the UK. Safety data from clinical trials spans over 1 year, showing good tolerability in most people. Like any prescription medication, risks and benefits must be weighed individually. Your GP or specialist will monitor you regularly if you're taking either medication to ensure it remains safe and effective for you.
Ready to take control of your weight loss journey?
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