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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How to Manage Mounjaro Side Effects
Mounjaro (tirzepatide) is a prescription-only GIP and GLP-1 receptor agonist that may help support weight loss, but like all medications, it can cause side effects. The most common side effects are nausea, diarrhoea, constipation, and stomach discomfort — particularly during the first few weeks and when doses increase. Most side effects improve over time as your body adjusts, but knowing how to manage them can make your treatment experience more comfortable. This guide covers practical strategies recommended by UK healthcare professionals to help reduce side effects while you're taking Mounjaro.
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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.
Common Side Effects and Management Strategies
Nausea and vomiting are very common, especially in the first 4 weeks. Try eating smaller, more frequent meals throughout the day rather than three large meals. Avoid fatty, greasy, or strongly flavoured foods — stick to bland options like toast, rice, chicken, and boiled vegetables. Stay hydrated with water or herbal tea. If nausea is severe or persistent beyond 2–3 weeks, speak to your prescriber about taking anti-nausea medication like metoclopramide or cyclizine.
Constipation and diarrhoea are both common as Mounjaro affects how quickly food moves through your stomach and intestines. For constipation, increase your water intake to at least 2 litres daily, eat high-fibre foods (vegetables, fruit, wholegrains), and take gentle exercise like walking. Over-the-counter laxatives like lactulose or docusate are safe options. For diarrhoea, stick to bland foods and avoid high-fat meals, which worsen symptoms. Both usually settle within 2–4 weeks as your gut adapts.
Stomach pain and indigestion may occur because Mounjaro slows gastric emptying. Eat slowly, chew thoroughly, and avoid lying down immediately after eating. Antacids or ranitidine may help if needed — check with your pharmacist or GP.
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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.
When to Seek Medical Advice
Most Mounjaro side effects are mild to moderate and improve naturally as your body adjusts. However, contact your GP or prescriber immediately if you experience: severe, persistent stomach pain that radiates to your back (possible pancreatitis); serious allergic reactions like facial swelling or difficulty breathing; vomiting that prevents you keeping food or fluids down for more than a few hours; or vision changes if you have diabetes.
Do not stop taking Mounjaro without medical advice. If side effects are significantly affecting your quality of life, your prescriber may suggest staying on your current dose longer before increasing, switching you to a different medication, or adjusting your diet and lifestyle approach. Keep a brief diary of side effects to share with your healthcare team — this helps them understand your individual experience and adjust your treatment plan accordingly.
- 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 usually last?
Most side effects peak in the first 1–2 weeks after starting or increasing your dose, then gradually improve over 2–4 weeks as your body adjusts. Nausea and stomach discomfort tend to settle first, while constipation or diarrhoea may take slightly longer. If side effects persist beyond 4 weeks at the same dose, discuss this with your prescriber.
Can I take anti-nausea medication while on Mounjaro?
Yes. Over-the-counter anti-nausea options like ginger, peppermint tea, or vitamin B6 may help. Prescription anti-nausea medications like metoclopramide or cyclizine are safe to use alongside Mounjaro — ask your GP or pharmacist. Avoid alcohol, which can worsen nausea.
What's the difference between Mounjaro and Wegovy side effects?
Both are injectable weight loss medications, but Mounjaro (tirzepatide) is a dual GIP/GLP-1 agonist while Wegovy (semaglutide) is a GLP-1 agonist only. Side effect profiles are similar (nausea, constipation, diarrhoea), but some people tolerate one better than the other. Your prescriber can advise which is most suitable for you.
Should I change my diet when I start Mounjaro?
Yes. When side effects are present, eat small, frequent bland meals and avoid fatty or greasy foods. Drink plenty of water and increase fibre gradually if you experience constipation. As side effects settle, you can return to a more varied, balanced diet — Mounjaro works best alongside a reduced-calorie diet and increased physical activity.
What if I'm having severe side effects — should I stop taking Mounjaro?
Never stop Mounjaro without medical advice, even if side effects are severe. Contact your prescriber instead — they may suggest staying on your current dose longer, taking anti-nausea medication, adjusting your diet, or trying a different approach. Stopping abruptly without guidance isn't recommended and may affect your weight loss progress.
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.