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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Understanding the Risks of Mounjaro in the UK
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK, but like all medications, it carries potential risks and side effects. This guide explains what you need to know about Mounjaro safety according to NHS and UK medical guidance, who shouldn't take it, and what side effects are most commonly reported in clinical trials. Understanding these risks helps you make an informed decision in consultation with your GP or a licensed 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 Side Effects and Safety Profile
The most common side effects of Mounjaro reported in clinical trials include nausea, diarrhoea, vomiting, constipation, and stomach pain. These tend to occur more frequently during the initial dose escalation phase and often improve as your body adjusts to the medication.
Nausea affects more than 1 in 10 people and is often the first side effect experienced. Gastrointestinal issues like diarrhoea and constipation are also very common but usually temporary. In clinical practice, many patients find these effects manageable with dietary adjustments and staying well hydrated.
Less common side effects include injection site reactions, fatigue, dizziness, headache, acid reflux, and hair loss. Serious side effects are rare but require immediate medical attention: severe stomach pain spreading to the back (possible pancreatitis), gallbladder problems, serious allergic reactions, or unexplained changes in vision.
Mounjaro is subject to additional MHRA monitoring (Black Triangle ▼ status), meaning any new safety signals are closely tracked by UK regulators.
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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
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Who Should Not Take Mounjaro and NHS Eligibility
Mounjaro is not suitable for everyone. You cannot take it if you are under 18 years old, pregnant, planning pregnancy, or breastfeeding. Anyone with a personal or family history of medullary thyroid carcinoma (a rare cancer) or multiple endocrine neoplasia type 2 (MEN2) must not take Mounjaro. People with severe stomach or digestion problems, serious allergies to tirzepatide, or uncontrolled diabetic retinopathy should discuss this carefully with their GP.
In the UK, Mounjaro eligibility typically requires a BMI of 30 or above, or 27 or above with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease. Lower BMI thresholds (27.5+) may apply if you are of South Asian, Chinese, Black African, or Black Caribbean heritage. Your prescriber will assess whether Mounjaro is right for you through a full clinical evaluation, including your medical history, current medications, and any existing health conditions.
Always inform your GP or prescriber of all medications you take — Mounjaro may interact with oral contraceptives and certain diabetes medications. Women using oral contraceptives should add a barrier method for 4 weeks after starting and after each dose increase, as Mounjaro may reduce oral medication absorption.
If you experience symptoms like severe abdominal pain, persistent vomiting, vision changes, rapid heartbeat, or signs of allergic reaction (swelling of lips, tongue, or throat), seek urgent medical attention immediately. Your prescriber should monitor you regularly, especially during dose escalation — this process of gradually increasing your dose (dose titration) is essential for tolerating the medication safely.
- 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 are the most common side effects of Mounjaro in the UK?
The most common side effects are nausea, diarrhoea, vomiting, and constipation. These typically occur during dose escalation and often improve as your body adjusts. Nausea affects more than 1 in 10 people. Most gastrointestinal side effects are temporary and manageable with dietary changes and hydration.
Can I take Mounjaro if I'm pregnant or breastfeeding?
No. Mounjaro is not recommended during pregnancy or while breastfeeding. If you're planning to become pregnant, discuss this with your GP or prescriber before starting treatment. Women of childbearing age should use effective contraception while taking Mounjaro.
How does Mounjaro compare to Wegovy in terms of safety and side effects?
Both Mounjaro and Wegovy are prescription-only GLP-1-based medications with similar common side effects (nausea, diarrhoea, constipation). Mounjaro is a dual GIP/GLP-1 agonist while Wegovy is GLP-1-only. Both require clinical assessment and monitoring. Individual tolerability varies; some people manage side effects better with one than the other.
What serious side effects should I watch for with Mounjaro?
Seek immediate medical attention if you experience severe stomach pain spreading to the back (possible pancreatitis), persistent vomiting, changes in vision, unexplained rapid heartbeat, or signs of allergic reaction like throat or lip swelling. These are rare but require urgent assessment by a healthcare professional.
Can I use the MyWeightCheck BMI calculator to check if I'm eligible for Mounjaro?
Yes. Calculate your BMI using our free BMI calculator on this page to see where you fall. However, BMI eligibility is only one factor — you'll also need assessment of weight-related health conditions and suitability based on your full medical history. Your GP or prescriber makes the final eligibility decision.
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.