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 — What UK Patients Need to Know
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK. Like all medicines, it can cause side effects. Most are mild and improve over time, but it's important to understand what to expect before starting treatment. This UK health guide explains the common and serious side effects, when to seek medical help, and how to manage them. Your GP or prescriber will assess whether Mounjaro is suitable for you based on your individual health circumstances.
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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) include nausea, diarrhoea, vomiting, constipation, and stomach pain. These are typically mild to moderate and often improve as your body adjusts to the medication, usually within a few weeks.
Nausea is the most frequently reported side effect, particularly when starting treatment or increasing doses. Eating smaller, lighter meals and staying hydrated can help manage this. Common side effects (affecting up to 1 in 10 people) include indigestion, acid reflux, injection site reactions (redness, bruising, or pain), fatigue, dizziness, and hair loss. Injection site reactions usually resolve within a few days. If side effects persist or significantly impact your quality of life, contact your GP or prescriber — they may adjust your dose or discuss alternatives.
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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.
Serious Side Effects — When to Seek Help
Whilst serious side effects are rare, you should seek urgent medical attention if you experience severe, persistent stomach pain spreading to your back (possible pancreatitis), serious allergic reactions (swelling of face, lips, tongue, or throat), changes in vision, or pain in the upper stomach area with yellowing of the skin. These require immediate assessment by your GP or A&E.
Always inform your prescriber of any existing medical conditions, especially gallbladder problems, pancreas disease, or history of medullary thyroid cancer. Use our free BMI calculator to understand your starting point, and consider tracking your progress with our weight loss calculator to monitor how you're responding to treatment alongside any side effects you experience.
- 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 is the most common Mounjaro side effect in the UK?
Nausea is the most frequently reported side effect, especially when starting treatment. It typically improves within the first few weeks as your body adjusts. Eating smaller meals, avoiding fatty foods, and staying hydrated can help manage nausea.
How long do Mounjaro side effects last?
Most common side effects improve within 2–4 weeks as your body adjusts. Injection site reactions usually resolve within days. If side effects persist beyond this period, contact your GP or prescriber for advice — they may adjust your dose.
Can I take other medications with Mounjaro?
Mounjaro may affect how your body absorbs oral medications. Inform your prescriber of all medications you take, especially oral contraceptives (you may need additional contraception for 4 weeks after starting or dose increases) and diabetes medications.
Should I stop Mounjaro if I get side effects?
Do not stop without medical advice. Many side effects improve naturally. Contact your GP or prescriber if side effects are severe, persistent, or affecting your wellbeing — they can advise whether to continue, adjust your dose, or explore alternatives.
What symptoms need emergency attention?
Seek urgent help for severe stomach pain spreading to your back, serious allergic reactions (swelling of face or throat), sudden vision changes, or yellowing of skin. These are rare but require immediate medical assessment. Always have your prescriber's contact details readily available.
Ready to take control of your weight loss journey?
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