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: Comparing Reactions and Side Effects
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections licensed in the UK, but they work differently and can cause different reactions. Understanding how each medication affects individuals helps your prescriber decide which is most suitable for you. This guide compares the side effects, clinical trial data, and key differences between these two GLP-1 and dual-mechanism options based on UK guidelines and evidence.
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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.
How Mounjaro and Wegovy Work Differently
Wegovy (semaglutide) is a GLP-1 receptor agonist — it activates a single hormone pathway in the gut that signals fullness and reduces appetite. Clinical trials showed Wegovy users lost an average of around 14.9% of body weight at the highest dose.
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two pathways simultaneously. This dual mechanism may explain why clinical trials showed Mounjaro users achieved average weight loss of 15–21% depending on dose. The difference in mechanism can influence which side effects you experience. Both require a clinical assessment before prescription, and both work best alongside reduced-calorie diet and increased physical activity.
- Wegovy: single mechanism (GLP-1 only)
- Mounjaro: dual mechanism (GIP + GLP-1)
- Both: prescription-only, once-weekly injections
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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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Comparing Side Effects: What the Evidence Shows
The most common reactions to both medications are gastrointestinal — nausea, diarrhoea, vomiting, and constipation affect more than 1 in 10 users. Individual reactions vary widely and often improve over time as the body adjusts. Mounjaro users in SURMOUNT trials reported slightly higher nausea rates early in treatment, whilst Wegovy users reported similar patterns in STEP trials. Both medications can cause injection site reactions, fatigue, and headache. Serious reactions (severe stomach pain, gallbladder problems, allergic responses) are uncommon but require urgent medical attention. Your GP or prescriber will monitor you throughout treatment and can adjust your dose if side effects are problematic.
- 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.
Which has fewer side effects, Mounjaro or Wegovy?
Both medications cause similar gastrointestinal side effects in clinical trials. Individual reactions vary significantly — some people tolerate one better than the other. A prescriber will assess your personal risk factors and medical history to choose the most suitable option for you.
Do Mounjaro and Wegovy cause nausea in everyone?
No. Gastrointestinal reactions like nausea are very common but not universal. Severity varies from mild discomfort to more troublesome symptoms. Most side effects typically improve after the first few weeks as your body adjusts to the medication.
Can you switch from Mounjaro to Wegovy if you have side effects?
Yes, switching between medications is possible under prescriber guidance, though this requires stopping one and starting the other. Your prescriber will assess whether switching is appropriate for you based on the type and severity of side effects you experienced.
What reactions mean I should stop taking Mounjaro or Wegovy?
Seek urgent medical attention if you experience severe stomach pain spreading to the back (possible pancreatitis), serious allergic reactions (swelling of face or throat), or persistent vomiting. Your GP will advise whether to continue or stop based on your symptoms.
Does the GLP-1 weight loss calculator help predict which medication's side effects I'll experience?
Our GLP-1 weight loss calculator estimates potential weight loss outcomes at different doses, but it cannot predict individual side effect profiles. Side effect experiences are highly individual — only your prescriber can assess your personal risk based on your medical history and current medications.
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.