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 Severe Side Effects — UK Access & Cost Guide
Mounjaro (tirzepatide) is a prescription-only weight management injection that has helped many UK adults with their weight loss journey. However, like all medicines, it can cause side effects — some common, and in rare cases, severe. Understanding the full picture of potential serious adverse effects, combined with practical information about UK prescription costs and access routes, helps you make an informed decision about whether this medication is right for you. This guide covers severe side effects, what to do if they occur, and how to access Mounjaro in the UK affordably.
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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.
Mounjaro Severe Side Effects and Emergency Warning Signs
While most people tolerate Mounjaro well, severe side effects are rare but require urgent medical attention. The most serious potential complications include acute pancreatitis (severe stomach pain spreading to the back), which may occur at any dose but is uncommon. Symptoms include sudden intense upper abdominal pain, nausea, and vomiting — seek emergency care immediately if this occurs.
Gallbladder problems including acute cholecystitis or choledocholithiasis can develop during tirzepatide treatment, typically causing sudden pain in the upper right abdomen. Serious allergic reactions, whilst extremely rare, present as swelling of the face, lips, tongue, or throat, and anaphylaxis requires emergency treatment.
Vision changes in people with type 2 diabetes may indicate diabetic retinopathy complications — notify your GP immediately. Severe dehydration, persistent vomiting lasting over a few hours, and unexplained fainting are also reasons to contact NHS 111 or attend A&E. Your prescriber will discuss these risks during your clinical assessment before starting Mounjaro.
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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.
UK Prescription Access: NHS vs Private Routes & Typical Costs
NHS access to Mounjaro for weight management is through tier 3 specialist weight management services following GP referral. NICE has approved tirzepatide within these services, though regional availability and waiting times vary significantly — some ICBs have lengthy waiting lists of 6–12 months.
Private prescription routes offer faster access without NHS waiting lists. Following an online clinical assessment with a UK-licensed prescriber, eligible patients receive a prescription dispensed by a GPhC-registered pharmacy. Typical monthly costs range from £150–£280 depending on dose — the Mounjaro KwikPen costs less per dose than Wegovy in many cases. Prices vary by provider and current dose. Compare providers carefully, ensure they are registered with the General Pharmaceutical Council, and use our GLP-1 weight loss calculator to estimate potential outcomes alongside cost.
- 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 should I do if I experience severe stomach pain on Mounjaro?
Severe, persistent abdominal pain spreading to the back may indicate pancreatitis and is a medical emergency. Stop Mounjaro immediately and contact NHS 999 or attend A&E without delay. This is rare but requires urgent assessment. Do not wait to contact your prescriber — seek emergency care first.
How much does Mounjaro cost on prescription in the UK?
Private prescription costs typically range from £150–£280 per month depending on your dose (2.5 mg to 15 mg). Each KwikPen contains 4 weekly injections. NHS costs are covered by the NHS once prescribed through specialist services, though you may face long waiting lists. Prices vary between providers — always check you're using a registered GPhC pharmacy.
Can I get Mounjaro through the NHS in the UK?
Yes, Mounjaro is NICE-approved for NHS specialist tier 3 weight management services. Access requires GP referral and assessment by a specialist multidisciplinary team. Waiting times vary by region (ICB) — some areas have several-month waits. Private prescription routes offer faster access without NHS waiting lists.
How do I know if I'm eligible for Mounjaro in the UK?
Generally, you may be eligible if you have a BMI of 30 or above, or BMI of 27–29.9 with weight-related health conditions (type 2 diabetes, high blood pressure, sleep apnoea, or cardiovascular disease). South Asian, Chinese, Black African, or Black Caribbean adults may have lower BMI thresholds. A licensed prescriber will assess your individual eligibility during a clinical consultation.
What's the difference between Mounjaro and Wegovy for accessing in the UK?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight management injections available through NHS specialist services or private prescription. Mounjaro is a dual GIP/GLP-1 agonist; Wegovy is a GLP-1 agonist only. Clinical trial data shows Mounjaro achieved slightly higher average weight loss (~21% vs ~15%), but individual results vary. Private costs are similar. Your prescriber will advise which is suitable for you.
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.