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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Who Should Not Take Mounjaro?
Mounjaro (tirzepatide) is a prescription-only injection for weight management, but it's not suitable for everyone. Certain health conditions, medications, and life circumstances mean you cannot or should not take it. If you're considering Mounjaro in the UK, your prescriber will assess your personal health to determine suitability. This guide explains who should avoid Mounjaro and how eligibility compares to Wegovy, the other main GLP-1 weight loss option available in the UK.
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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.
Absolute Contraindications — Who Cannot Take Mounjaro
Some people must not take Mounjaro under any circumstances. You cannot take Mounjaro if you:
- Are under 18 years of age (not licensed for children)
- Are pregnant or planning to become pregnant within the next few months
- Are breastfeeding — tirzepatide may pass into breast milk
- Have a personal or family history of medullary thyroid carcinoma (a rare thyroid cancer)
- Have a history of Multiple Endocrine Neoplasia syndrome type 2 (MEN2)
- Have severe gastroparesis or other serious stomach/digestive problems
- Have had a serious allergic reaction to tirzepatide or any ingredients in Mounjaro KwikPen
Your prescriber will ask detailed questions about your medical history to identify these contraindications before issuing a Mounjaro UK prescription. If any apply to you, discuss alternative weight management options with your GP, including Wegovy or other evidence-based approaches.
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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.
Relative Cautions — When Mounjaro Requires Extra Care
You may still be able to take Mounjaro, but your prescriber needs to know about these conditions and may need to adjust your treatment. Caution is needed if you have:
- Type 2 diabetes or are taking insulin or sulphonylureas (medication interaction risk)
- Severe kidney disease or on dialysis
- Severe liver disease
- Pancreatitis history or risk factors
- Gallbladder problems or bile duct disease
- Eye problems from diabetes (retinopathy) — may worsen temporarily with gradual weight management
- Heart disease or history of heart attack or stroke
A clinical assessment will help your prescriber weigh the risks and benefits for you personally. You may need dose adjustments, more frequent monitoring, or Mounjaro may be unsuitable. Use our GLP-1 weight loss calculator to explore if medication might support your goals, but always discuss these medical factors with your healthcare team first.
- 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.
Can I take Mounjaro if I'm pregnant or breastfeeding?
No. Mounjaro must not be used during pregnancy — it may harm the developing baby. If you're planning pregnancy, stop Mounjaro at least 3 months before conception. It's also not safe while breastfeeding. Discuss timing and alternatives with your GP if you want to start a family.
What's the difference between Mounjaro and Wegovy eligibility criteria?
Both Mounjaro and Wegovy have similar absolute contraindications (pregnancy, allergy, medullary thyroid cancer history, MEN2). However, they're different medications: Mounjaro is a dual GIP+GLP-1 agonist; Wegovy is GLP-1 only. Clinical trial data differs (SURMOUNT for Mounjaro, STEP trials for Wegovy), and individual suitability depends on your specific health profile and prescriber assessment.
If I have type 2 diabetes, can I take Mounjaro for weight loss?
Yes, many people with type 2 diabetes can take Mounjaro — it's actually licensed for both weight management and blood sugar control. However, if you're on insulin or sulphonylurea medication, you may need dose adjustments because Mounjaro affects blood sugar. Your GP or prescriber will monitor you more closely and manage this risk.
What should I do if I think Mounjaro might not be safe for me?
Book an appointment with your GP or use a UK-licensed online prescriber for a clinical assessment. Be honest about all your medical conditions, medications, and allergies. They'll review your full health picture and either approve Mounjaro, suggest Wegovy, or recommend other weight management options. Never start without a prescription and professional evaluation.
Can I use the BMI calculator to check if I'm eligible for Mounjaro?
The BMI calculator will show your personal BMI and NHS weight categories, which is the first eligibility step. However, BMI alone doesn't determine if Mounjaro is safe for you — that requires a prescriber to assess your full medical history, medications, and any contraindications. Use the calculator as a starting point, then discuss options with your healthcare team.
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.