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 Side Effects — Complete UK Guide
Mounjaro (tirzepatide) is a prescription-only weight management injection now available through NHS specialist services and private prescribing in the UK. Like all medicines, it can cause side effects. Most are mild and manageable, though some require medical attention. Understanding what to expect helps you use the medication safely and recognise when to contact your GP. This guide covers NHS-approved information about Mounjaro side effects in UK adults.
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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.
Common Mounjaro Side Effects in the UK
Very common side effects (affecting more than 1 in 10 people using Mounjaro) include nausea, diarrhoea, vomiting, constipation, and stomach pain. These typically occur early in treatment or when the dose increases, and often improve within days or weeks as your body adjusts. Nausea is the most frequently reported side effect.
- Nausea — starts within hours of injection, usually settles within 24–48 hours
- Diarrhoea — more common than constipation but both can occur
- Stomach pain — mild to moderate, usually resolves quickly
- Vomiting — less common than nausea, requires hydration monitoring
Common side effects (affecting up to 1 in 10 people) include indigestion, acid reflux, injection site reactions (redness or bruising), fatigue, dizziness, hair loss, and decreased appetite. Your GP can advise on managing these. NHS guidance suggests staying hydrated and eating small, frequent meals. If side effects persist beyond two weeks or significantly impact daily life, contact your GP for support — prescription adjustments may help.
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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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Serious Side Effects — When to Seek Medical Attention
Whilst uncommon, some side effects require urgent medical attention. Contact your GP immediately or call NHS 111 if you experience severe stomach pain spreading to your back (possible pancreatitis), serious allergic reactions (swelling of face, lips, tongue, or throat), changes in vision, or persistent vomiting or dehydration. Gallbladder problems can cause upper abdominal pain and may need specialist assessment. If you have a personal or family history of thyroid cancer, inform your prescriber before starting Mounjaro. Report any unusual symptoms to your GP rather than stopping the injection without medical guidance.
When taking Mounjaro via NHS tier 3 weight management services, you'll receive regular check-ups where side effects are monitored. Use our free BMI calculator and GLP-1 weight loss calculator to track your progress alongside medical supervision.
- 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's the difference between Mounjaro and Wegovy side effects?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, whilst Wegovy (semaglutide) is a GLP-1 agonist only. Both cause similar gastrointestinal side effects (nausea, diarrhoea), but Mounjaro may have a slightly different side effect profile due to its dual mechanism. Individual responses vary. Your GP can discuss which is more suitable for you.
How long do Mounjaro side effects last?
Most common side effects settle within a few days to two weeks as your body adjusts. Nausea typically improves after 24–48 hours. If side effects persist beyond two weeks or worsen, contact your GP. Dose titration (gradual increases) helps minimise side effects — you start at 2.5mg and increase every 4 weeks.
Can I stop Mounjaro if side effects are too severe?
Do not stop without medical guidance. Contact your GP instead. They may pause your dose, switch you to a lower dose, or recommend alternative treatments. Stopping suddenly without support isn't recommended. Your prescriber can adjust your treatment plan based on your experience.
Are Mounjaro side effects worse than other weight loss injections?
Side effect profiles are similar across GLP-1 and GIP/GLP-1 medications. Mounjaro's dual mechanism may help some people but isn't "worse" or "better" — it's individual. Nausea and digestive issues are common with all weight loss injections. Your GP will assess which option suits you best.
Should I tell my GP about minor side effects?
Yes. Even mild side effects are worth mentioning at your next appointment. Your GP can offer practical tips (small meals, staying hydrated) and monitor whether side effects improve over time. If they're affecting your quality of life, treatment adjustments may help. Regular contact with your healthcare team is important.
Ready to take control of your weight loss journey?
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