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 — What UK Patients Need to Know
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK. Whilst most side effects are mild to moderate, some serious adverse reactions can occur and require urgent medical attention. This guide explains the potentially dangerous side effects you should be aware of, based on NHS guidance and clinical trial data, and when to contact your GP or seek emergency care. Understanding these risks helps you use the medication safely and recognise warning signs early.
10,000+ calculations today · NHS-aligned thresholds · 100% free
Calculate Your BMI Instantly
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 Immediate Medical Attention
Several serious side effects of tirzepatide (Mounjaro's active ingredient) require urgent medical attention and should not be ignored:
- Severe pancreatitis — sudden, severe pain in the upper abdomen spreading to your back, nausea, vomiting, and elevated pancreatic enzymes. This is a medical emergency.
- Gallbladder problems — severe pain in the upper right abdomen, jaundice (yellowing of skin/eyes), and fever. gradual weight management increases gallstone risk.
- Serious allergic reactions — swelling of face, lips, tongue, or throat; difficulty breathing; rash or hives. Seek immediate emergency care (call 999).
- Severe dehydration — persistent vomiting or diarrhoea leading to dizziness, confusion, or rapid heartbeat.
- Thyroid changes — though rare, monitor for persistent hoarseness, difficulty swallowing, or neck lumps, especially if there is family history of thyroid cancer.
The UK MHRA (Medicines and Healthcare products Regulatory Agency) monitors Mounjaro safety continuously and all serious adverse events must be reported.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
NHS Safety Guidance and When to Seek Help
Your NHS prescriber will provide safety information before starting Mounjaro and will monitor you during treatment through your GP or specialist weight management service. If you experience any of the serious symptoms listed above, stop using Mounjaro immediately and seek emergency care by calling 999 or attending your nearest A&E. For less severe but concerning symptoms — such as persistent severe nausea, unexplained weight loss beyond expected, changes in vision, or heart palpitations — contact your GP or the NHS 111 service for urgent advice. Always report any unexpected symptoms to your prescriber. Individual risk varies based on your medical history, medications, and other health conditions, which is why the initial clinical assessment is essential before Mounjaro is prescribed.
- 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 compared to other weight loss injections?
Mounjaro and Wegovy (semaglutide) have similar safety profiles — both are GLP-1/GIP medications with comparable serious side effect rates. The main difference is mechanism: Mounjaro is dual GIP + GLP-1, whilst Wegovy is GLP-1 only. Individual risk depends on your health conditions, not the medication alone. Your GP will assess which is safer for you.
Can Mounjaro cause pancreatitis?
Yes, acute pancreatitis is a documented serious side effect, though rare. It presents as severe, sudden upper abdominal pain spreading to the back, with nausea and vomiting. If you experience this, stop Mounjaro immediately and call 999. Pancreatitis risk is higher in people with personal or family history of pancreatitis or gallstones.
What should I do if I experience severe nausea or vomiting on Mounjaro?
Mild to moderate nausea is very common, especially when starting or increasing doses, and often settles within days. However, if vomiting is severe, persistent, or preventing you from keeping food or fluids down, contact your GP or NHS 111. You may need to pause your dose or switch medications. Do not stop suddenly without medical advice.
How is Mounjaro monitored for safety in the UK?
If prescribed through NHS specialist services, you'll have regular GP or clinic appointments to monitor side effects, weight loss, and blood sugar. The MHRA monitors all adverse events reported by healthcare professionals and patients. Private prescribers also require follow-up monitoring. Always report side effects to your prescriber — do not manage concerning symptoms alone.
Can Mounjaro cause heart problems?
Serious heart problems are not a documented side effect of tirzepatide. However, gradual weight management can sometimes affect blood pressure or heart rate. If you experience palpitations, chest pain, shortness of breath, or fainting, seek immediate medical attention. Your prescriber will review your cardiovascular health before starting Mounjaro, especially if you have existing heart conditions.
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.