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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What Are the Health Risks of Mounjaro?
Mounjaro (tirzepatide) is a prescription-only weight loss injection that works as a dual GIP and GLP-1 receptor agonist. While clinical trials showed it can help with weight management, like all prescription medications, it carries potential health risks and side effects that you should understand before starting treatment. This guide explains the serious and common risks associated with Mounjaro in the UK, helping you make an informed decision with your prescriber.
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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
The most common side effects of Mounjaro affect the digestive system. Nausea is reported by more than 1 in 10 people, particularly in the first few weeks of treatment or when doses increase. Other frequent side effects include diarrhoea, vomiting, constipation, and stomach pain. These typically improve as your body adjusts, though some people experience them throughout treatment.
Serious side effects requiring urgent medical attention include severe, persistent stomach pain spreading to your back (which may indicate pancreatitis), gallbladder problems causing upper stomach pain, serious allergic reactions with facial or throat swelling, and changes in vision. Some people experience hair loss (alopecia) or fatigue. Injection site reactions like redness, bruising, or pain at the injection site are common but usually mild. Individual tolerance varies significantly—always inform your prescriber if side effects persist or worsen.
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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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Understanding Mounjaro Risks vs Benefits
Before starting Mounjaro, your prescriber will assess whether the potential health benefits outweigh the risks for your individual situation. The medication may not be suitable if you have a history of pancreatitis, severe gastroparesis, thyroid cancer, or medullary thyroid carcinoma. Pregnant women, those planning pregnancy, and breastfeeding mothers should not use Mounjaro.
Unlike some GLP-1 alternatives like Wegovy, Mounjaro is a dual-mechanism injection, which means it activates two hormone pathways simultaneously. This dual action may increase effectiveness for some people but also carries specific risk considerations. Your GP or licensed prescriber can explain how your personal health history affects your suitability. Use our free BMI calculator above to understand your current health category, and discuss all concerns with your healthcare provider before treatment begins.
- 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 dangerous?
Mounjaro is not inherently dangerous when prescribed appropriately and used as directed. However, like all prescription medications, it carries risks and potential side effects. Serious complications are rare but can include pancreatitis. Most side effects are mild and improve over time. Your prescriber assesses whether benefits outweigh risks for your individual health situation before prescribing.
What's the difference between Mounjaro risks and Wegovy risks?
Both are GLP-1-based weight loss injections, but Mounjaro is a dual GIP and GLP-1 agonist, whereas Wegovy is GLP-1 only. Side effect profiles are similar—both commonly cause nausea and digestive issues. Mounjaro uses a different dose titration schedule and reaches a higher maximum dose (15mg vs 2.4mg). Individual risk tolerance differs; your prescriber will recommend the most suitable option for you.
Can Mounjaro cause pancreatitis?
Pancreatitis (inflammation of the pancreas) is a rare but serious potential side effect of GLP-1 and GIP/GLP-1 medications. Seek immediate medical attention if you experience severe, persistent stomach pain radiating to your back, as this may indicate pancreatitis. People with a history of pancreatitis should not use Mounjaro. Always inform your prescriber of any previous pancreas problems before starting treatment.
Who should not take Mounjaro?
Mounjaro is not suitable for people under 18, those pregnant or breastfeeding, people with a personal or family history of medullary thyroid carcinoma, those with severe gastroparesis, or anyone with known allergy to tirzepatide. People with a history of pancreatitis should not use it. Your prescriber will review your full medical history to assess suitability during your clinical assessment.
How long do Mounjaro side effects last?
Most common side effects like nausea, diarrhoea, and constipation improve within the first few weeks as your body adjusts. Some people experience them throughout treatment, particularly when doses increase. If side effects persist beyond a few weeks or are severe, contact your prescriber—dosing adjustments or alternative medications may be considered. Serious side effects require immediate medical attention.
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