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 Side Effects of Mounjaro Injections in the UK?
Mounjaro (tirzepatide) is a prescription-only weekly weight loss injection licensed for use in the UK. Like all medicines, Mounjaro can cause side effects. Most are mild and tend to improve over time as your body adjusts, but it's important to understand what to expect and when to seek medical advice from your GP. This guide covers the most common reactions reported in clinical trials and what NHS guidance recommends.
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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.
Most Common Mounjaro Side Effects
In clinical trials, the most frequently reported side effects of Mounjaro were nausea, diarrhoea, vomiting, and constipation. These gastrointestinal effects occur in more than 1 in 10 people and are most common when starting the injection or increasing your dose.
- Nausea: Often mild and typically decreases after the first few weeks. Eating smaller, lighter meals may help.
- Diarrhoea: Can occur due to how Mounjaro affects stomach emptying. Usually resolves within days to weeks.
- Vomiting: Less common than nausea but may happen, especially if nausea is severe.
- Constipation: Some people experience this instead of diarrhoea. Increase fluids and fibre intake.
- Stomach pain: Mild abdominal discomfort is common, but severe pain needs urgent GP assessment.
Other common effects include indigestion, headache, fatigue, dizziness, injection site reactions (redness or bruising), and hair loss (alopecia). These typically improve as your body adjusts to the weekly weight loss injection.
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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.
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Serious Side Effects and When to Seek Urgent Care
Seek immediate medical attention if you experience:
- Severe stomach pain spreading to your back: May indicate pancreatitis. This is a medical emergency.
- Signs of allergic reaction: Swelling of face, lips, tongue, or throat; difficulty breathing.
- Gallbladder problems: Sudden pain in the upper stomach area or back, especially with nausea.
- Vision changes: Particularly important if you have diabetes — may signal retinopathy complications.
- Rapid heart rate or chest pain: Seek emergency care immediately.
Contact your GP immediately if side effects don't improve after a few weeks, become severe, or are affecting your ability to manage daily life. Your prescriber may adjust your Mounjaro dose or suggest alternative options.
- 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.
How long do Mounjaro side effects last?
Most side effects improve within 1–2 weeks as your body adjusts. Nausea and gastrointestinal symptoms are most noticeable in the first few days after injection, then typically settle. If side effects persist beyond 3–4 weeks, contact your GP for guidance.
Can I reduce Mounjaro side effects by eating differently?
Yes. Eating smaller, more frequent meals; avoiding fatty or spicy foods; staying hydrated; and eating slowly may help reduce nausea and stomach discomfort. Increasing fibre and water intake helps with constipation. Your GP or a registered dietitian can offer personalised dietary advice.
Is nausea a sign that Mounjaro isn't working for me?
No. Nausea is a common adjustment reaction and doesn't mean the medication isn't effective. In fact, nausea often improves while weight loss continues. However, if nausea is severe or you cannot manage it, your prescriber can adjust your dose or explore alternatives.
What should I do if I have an allergic reaction to Mounjaro?
Allergic reactions are rare but serious. Symptoms include swelling of the face, lips, or throat; difficulty breathing; or severe rash. Call 999 or go to A&E immediately. Always inform healthcare staff that you're taking Mounjaro and report any reaction to your GP and prescriber.
Are Mounjaro side effects the same as Wegovy side effects?
Mounjaro and Wegovy (the weekly weight loss injection semaglutide) cause similar gastrointestinal side effects because both are GLP-1-based medications. However, Mounjaro is a dual GIP/GLP-1 receptor agonist, so individual side effect profiles may vary slightly. Your prescriber can discuss which is more appropriate for you.
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