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 Side Effects — Complete UK Health Guide
Mounjaro (tirzepatide) is a prescription-only weight management injection that can cause side effects, ranging from mild nausea to more serious complications. Understanding what to expect is important before starting treatment. This guide covers all known side effects based on clinical trial data, what's considered normal, and when to seek medical attention. Always consult your GP before starting any prescription medication.
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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.
What Are the Most Common Mounjaro Side Effects?
Very common side effects (affecting more than 1 in 10 people) include nausea, diarrhoea, vomiting, and constipation — these are the most frequently reported effects during the dose titration phase. Stomach pain and indigestion are also common, particularly when starting or increasing doses. These effects typically improve over time as your body adjusts to the medication. Common side effects (affecting up to 1 in 10 people) include acid reflux, fatigue, dizziness, injection site reactions (redness or bruising), and hair loss. Many people find that gastrointestinal symptoms reduce significantly after the first few weeks of treatment, especially once the dose stabilises at the maintenance level. If you're experiencing persistent nausea or vomiting, speak to your prescriber about whether a dose adjustment or slower titration schedule might help.
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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.
Serious Side Effects — When to Seek Urgent Medical Help
While serious side effects are uncommon, you should seek immediate medical attention if you experience severe, persistent stomach pain spreading to your back (which may indicate pancreatitis), serious allergic reactions including swelling of the face, lips, tongue, or throat, changes in vision (which could signal diabetic retinopathy complications), or gallbladder problems such as pain in the upper stomach area. These symptoms require urgent medical assessment and should not be ignored. Call 999 or attend A&E immediately if you experience any of these warning signs. Your GP or prescriber should also be informed of any new or worsening symptoms during treatment, even if they don't feel immediately serious.
- 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 nausea with Mounjaro normal and will it go away?
Yes, nausea is very common when starting Mounjaro and typically affects more than 1 in 10 people. It usually improves within the first few weeks as your body adjusts. If nausea persists or is severe, contact your prescriber — they may adjust your dose or suggest dietary changes. Most people find symptoms reduce significantly by week 4–6 of treatment.
Can Mounjaro cause pancreatitis or gallbladder problems?
Pancreatitis is a serious but rare side effect. Seek immediate medical attention if you experience severe, persistent stomach pain radiating to your back. Gallbladder problems are also possible. These serious symptoms require urgent GP or A&E assessment and should never be ignored, even if unlikely.
What's the difference between common and serious Mounjaro side effects?
Common side effects like nausea, diarrhoea, and constipation usually settle within weeks and aren't dangerous. Serious side effects like severe stomach pain, allergic reactions, or vision changes require immediate medical attention. Always report any new symptoms to your prescriber promptly.
How does Mounjaro's side effect profile compare to Wegovy?
Both Mounjaro and Wegovy cause similar gastrointestinal side effects due to their GLP-1 activity. However, Mounjaro uses a dual GIP and GLP-1 mechanism, while Wegovy is GLP-1 only. Clinical trials show comparable safety profiles, though individual responses vary. Your prescriber can discuss which medication might suit you best.
Should I stop taking Mounjaro if I experience side effects?
Don't stop suddenly without medical guidance. Most side effects improve with time or dose adjustment. Contact your prescriber if effects are severe or persistent — they can modify your treatment plan. Never adjust or stop prescription medication without professional advice, as this can affect your health and weight management progress.
Ready to take control of your weight loss journey?
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