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
Mounjaro Dangerous Side Effects — UK Safety Guide
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK. While clinical trials have shown it can help with weight loss, like all medications it carries potential side effects. This guide explains the serious side effects you should be aware of, what symptoms to watch for, and when to seek urgent medical attention. Understanding these risks helps you use the medication safely alongside your GP's guidance.
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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.
Serious Side Effects That Require Urgent Medical Attention
Pancreatitis (inflammation of the pancreas) is the most serious potential side effect. Seek emergency medical attention immediately if you experience severe, persistent stomach pain that spreads to your back, especially accompanied by nausea and vomiting. This is rare but requires urgent assessment.
Serious allergic reactions may occur, though uncommon. Signs include swelling of the face, lips, tongue, or throat; difficulty breathing; or severe rash. Use your emergency inhaler if you have one and call 999 immediately.
Gallbladder complications can develop, causing severe pain in the upper stomach area, often after eating fatty foods. Contact your GP urgently if this occurs.
Changes in vision may indicate diabetic retinopathy complications in those with diabetes. Report any vision changes to your GP immediately.
Severe dehydration from persistent vomiting or diarrhoea can become dangerous. Drink small, frequent amounts of fluids and contact your GP if symptoms persist for more than a few hours.
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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.
Common Side Effects and Risk Management
Most people using Mounjaro experience mild to moderate side effects that typically improve within days or weeks. Nausea, diarrhoea, and constipation are very common, especially when starting or increasing doses. These usually settle as your body adjusts.
To manage common side effects: eat small, frequent meals; stay hydrated; avoid fatty or greasy foods; and increase physical activity gradually. Your prescriber may recommend dose adjustments if side effects are significant. Always discuss persistent symptoms with your GP rather than stopping the medication abruptly. Use our free GLP-1 weight loss calculator to understand what to expect, and track your progress and any side effect patterns using our progress tracker to share with your healthcare provider.
- 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.
Is Mounjaro safe to use in the UK?
Yes — Mounjaro (tirzepatide) is approved and licensed by the MHRA in the UK for weight management. It is subject to additional safety monitoring (Black Triangle ▼ status). Serious side effects are rare, but you must receive a prescription and clinical assessment from a UK-licensed prescriber before use.
What's the difference between dangerous and common side effects?
Common side effects (nausea, diarrhoea, stomach pain) are usually mild and improve over time. Dangerous side effects are rare but require immediate medical attention — such as severe stomach pain suggesting pancreatitis, or difficulty breathing from allergic reaction. Know the difference and seek urgent help if you experience severe symptoms.
Can Mounjaro cause pancreatitis?
Pancreatitis is a rare but serious potential side effect of GLP-1 and GIP/GLP-1 medications. Seek emergency medical attention if you experience sudden, severe stomach pain spreading to your back, nausea, and vomiting. Your GP can assess whether Mounjaro is appropriate for you based on your individual health history.
What should I do if I experience side effects?
For mild side effects, stay hydrated, eat small meals, and wait a few days as symptoms often improve. For severe or persistent symptoms, contact your GP immediately. Never stop Mounjaro abruptly without medical guidance — your prescriber may adjust your dose or recommend alternative management.
Are people with certain conditions at higher risk from Mounjaro?
Yes — your prescriber will assess your suitability before prescribing. Those with a personal or family history of medullary thyroid cancer, Multiple Endocrine Neoplasia syndrome, severe gastroparesis, or certain kidney/liver conditions may not be eligible. Always disclose your full medical history to your prescriber.
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.