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 Worst Side Effects of Mounjaro?
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK that can help support weight loss through its dual GIP and GLP-1 mechanism. While many people tolerate it well, understanding the more serious side effects is important before starting treatment. Most side effects are mild to moderate, but some require urgent medical attention. This guide explains which Mounjaro side effects are most serious and when you should contact your GP or seek emergency care.
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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.
Serious Side Effects That Need Urgent Attention
While common side effects like nausea and constipation usually settle within a few weeks as your body adjusts, some side effects of Mounjaro demand immediate medical attention. These include severe, persistent stomach pain spreading to your back — a possible sign of pancreatitis, inflammation of the pancreas that requires emergency care. Serious allergic reactions such as swelling of your face, lips, tongue, or throat need immediate attention at an A&E department.
- Severe stomach pain radiating to the back (possible pancreatitis)
- Difficulty breathing or throat swelling (allergic reaction)
- Sudden changes in vision (diabetic retinopathy complications)
- Severe dehydration from persistent vomiting or diarrhoea
- Signs of gallbladder problems — severe upper abdominal pain, fever, yellowing of skin
- Rapid or severe changes in heart rate
Your prescriber should provide guidance on which symptoms warrant emergency care. If you experience any of these, contact 999 or go to A&E immediately rather than waiting for a GP appointment. These serious side effects are rare but recognising them quickly can be lifesaving.
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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.
Managing Side Effects and NHS Guidance
Most people experience common side effects like nausea, diarrhoea, constipation, or stomach discomfort, particularly when starting or increasing your Mounjaro dose with the KwikPen. These typically improve within days or weeks as your body adapts. Dose titration — gradually increasing from 2.5mg up to 15mg over several weeks — helps your body adjust and reduces the severity of initial side effects.
If you're prescribed Mounjaro through NHS tier 3 specialist services, your prescriber will monitor you closely and discuss managing side effects. They may adjust your dose, slow your titration schedule, or suggest practical strategies like eating smaller, frequent meals to reduce nausea. Staying hydrated and maintaining regular physical activity also helps manage gastrointestinal side effects. Never stop taking Mounjaro suddenly without speaking to your prescriber first.
Use our free GLP-1 weight loss calculator to understand potential outcomes, and track your progress and side effect timeline with our progress tracker.
- 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 Mounjaro cause pancreatitis?
Yes, pancreatitis is a rare but serious side effect reported with GLP-1 and GIP/GLP-1 medications like Mounjaro. Seek emergency care immediately if you develop severe stomach pain spreading to your back, fever, or persistent vomiting. The MHRA monitors this side effect closely — always report new symptoms to your GP.
What's the difference between common and serious Mounjaro side effects?
Common side effects (nausea, diarrhoea, constipation, stomach pain) usually settle within weeks as your body adjusts. Serious side effects (severe pain, allergic reactions, vision changes, severe dehydration) require urgent medical attention and are rarer but potentially dangerous.
Will side effects stop if I continue taking Mounjaro?
For most people, yes. Common side effects typically improve significantly within 4–8 weeks as your body adapts. Slower dose titration with the KwikPen can help reduce initial side effects. Your prescriber can adjust your schedule if needed based on how you're tolerating the medication.
What should I do if I have severe nausea or vomiting on Mounjaro?
Contact your GP or prescriber. Severe, persistent nausea may mean your dose is too high for your current tolerance. Your prescriber may slow your titration or reduce your current dose. Eating smaller, frequent meals and staying hydrated helps manage mild to moderate nausea while your body adjusts.
Is Mounjaro safe if I have a history of gallbladder problems?
Inform your prescriber about any gallbladder history before starting Mounjaro. Weight loss medications can increase gallbladder complications risk. Your prescriber will assess whether Mounjaro is appropriate for you or if an alternative approach is safer. Never start treatment without full disclosure of your medical history.
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.