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
Free tracker included with every download this week
What Are the Side Effects of Mounjaro?
Mounjaro (tirzepatide) is a prescription-only GLP-1 and GIP receptor agonist injection used for weight management in the UK. Like all medicines, it can cause side effects, though not everyone experiences them. Understanding what to expect helps you work with your prescriber to manage them effectively. This guide covers side effects in an NHS and UK healthcare context, helping you make an informed decision about whether this medication is right for you.
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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 and Serious Side Effects of Mounjaro
Very common side effects (affecting more than 1 in 10 people) include nausea, diarrhoea, vomiting, constipation, and stomach pain. These occur because Mounjaro slows how quickly food moves through your digestive system. Common side effects (up to 1 in 10 people) include indigestion, acid reflux, injection site reactions like redness or bruising, fatigue, dizziness, and hair loss.
- Nausea and vomiting are often most severe in the first few weeks and typically improve as your body adjusts
- Gastrointestinal side effects can often be managed by eating smaller meals, avoiding fatty foods, and staying hydrated
- Injection site reactions are usually mild and resolve within a few hours
Serious side effects requiring immediate medical attention include severe stomach pain spreading to the back (possible pancreatitis), allergic reactions, gallbladder problems, or changes in vision. Tell your GP immediately if you experience any of these.
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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.
Managing Side Effects and NHS Support
Most people find side effects improve within 2–4 weeks as their body adjusts to the medication. Your prescriber may suggest practical steps: eating smaller portions, staying hydrated, avoiding high-fat foods, and taking anti-sickness medication if needed. If side effects become unbearable, your GP or weight management specialist can adjust your dose or suggest alternative treatments. The NHS notes that benefits of weight loss often outweigh temporary discomfort for many patients, but individual tolerance varies significantly.
- Keep a symptom diary to track patterns and discuss with your prescriber
- Contact your GP if side effects persist beyond 4 weeks or worsen
- Use our BMI calculator to monitor your progress and health context alongside medication
- NHS tier 3 weight management services provide ongoing support throughout treatment
- 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.
Are Mounjaro side effects permanent?
No. Most side effects are temporary and improve within 2–4 weeks as your body adjusts. Serious side effects are rare, but you should report any persistent symptoms to your GP immediately.
Can you reduce Mounjaro side effects by eating differently?
Yes. Eating smaller, frequent meals, avoiding fatty or greasy foods, staying hydrated, and limiting sugar can significantly reduce nausea and digestive side effects. Your prescriber or a dietitian can provide personalised advice.
Is Mounjaro safe according to MHRA and NHS guidance?
Yes. Mounjaro is approved by the MHRA for weight management in UK adults and is used within NHS specialist weight management services. It has a standard safety monitoring status. Your prescriber will assess whether it's suitable for you based on your individual health.
What's the difference between common and serious side effects?
Common side effects (nausea, diarrhoea, constipation) are temporary and manageable. Serious side effects (severe stomach pain, allergic reactions, vision changes) are rare but require immediate medical attention. Contact your GP right away if you experience serious symptoms.
How do Mounjaro side effects compare to Wegovy?
Both medications cause similar gastrointestinal side effects, though Mounjaro's dual mechanism (GIP and GLP-1) may produce different patterns. Individual experiences vary significantly. Your prescriber can discuss which medication suits you based on your health history and tolerance to side 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.