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
What Are the Side Effects of Taking Mounjaro?
Mounjaro (tirzepatide) is a prescription-only GIP and GLP-1 receptor agonist injection used for weight management in UK adults. Like all medications, it can cause side effects. Most are mild to moderate and improve over time, but it's important to understand what to expect before starting treatment. This guide covers the most common side effects, when they typically occur, and what action to take if you experience serious symptoms.
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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 Managing Them
The most frequently reported side effects of Mounjaro affect the digestive system. Nausea is the most common, occurring in more than 1 in 10 people, and often improves as your body adjusts to the medication over the first few weeks. Diarrhoea, constipation, and stomach pain are also very common and typically reduce with time.
- Start with small, regular meals rather than large portions
- Stay well hydrated throughout the day
- Avoid fatty, greasy, or spicy foods initially
- Take time eating and chew food thoroughly
Other common side effects include indigestion, fatigue, dizziness, and headache. Injection site reactions—redness, bruising, or mild pain at the injection point—are also reported. Most side effects are temporary and manageable. Your GP or prescriber can advise you on over-the-counter remedies like ginger for nausea or antacids for indigestion. Calculate your potential weight loss journey with our free GLP-1 weight loss calculator to see if the expected benefits outweigh temporary side effects.
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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 and When to Seek Help
While rare, some side effects require urgent medical attention. Severe stomach pain spreading to your back could indicate pancreatitis—stop Mounjaro immediately and contact your GP or go to A&E. Serious allergic reactions (swelling of face, lips, tongue, or throat, difficulty breathing) require emergency care.
Other serious warning signs include sudden changes in vision (especially if you have diabetes), gallbladder problems (persistent pain in your upper stomach area), severe vomiting, or rapid heart rate changes. Always inform your prescriber of any existing health conditions before starting Mounjaro. Side effects are usually temporary, but your GP should monitor you regularly throughout your treatment. Use our free BMI calculator to track your overall health progress alongside medical guidance.
- 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, particularly nausea and digestive issues, improve within the first 1–2 weeks as your body adjusts. Some people experience improvement within days. If side effects persist beyond 4 weeks or worsen, contact your GP—your dose may need adjusting, or Mounjaro may not be suitable for you.
Can I take over-the-counter medicine to manage Mounjaro side effects?
Yes, many people use paracetamol for headaches or antacids for indigestion. However, always check with your GP or pharmacist before taking additional medicines, as some may interact with Mounjaro or reduce absorption of oral contraceptives.
What's the difference between Mounjaro and Wegovy side effects?
Both are GLP-1 receptor agonists causing similar digestive side effects, though Mounjaro is a dual GIP and GLP-1 agonist which may produce a slightly different profile. Individual experiences vary. Your prescriber can discuss which medication suits you best during your clinical assessment.
Do side effects get worse at higher Mounjaro doses?
Side effects can temporarily increase when moving to a higher dose with the KwikPen, but typically settle within 1–2 weeks. Your prescriber increases doses gradually over 16+ weeks to minimise this. Never increase your dose without medical guidance.
Should I stop Mounjaro if I experience side effects?
Don't stop without contacting your GP first. Most side effects are manageable and improve over time. However, seek immediate medical help for severe stomach pain, allergic reactions, or vision changes. Your prescriber may adjust your dose, switch medication, or provide management strategies rather than stopping treatment entirely.
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.