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 Mounjaro?
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK. Like all medicines, it can cause side effects. Most side effects are mild to moderate and tend to improve over time as your body adjusts, but it's important to understand what to expect before starting treatment. This guide covers the side effects reported in clinical trials and NHS guidance, helping you make an informed decision with your GP.
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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 of Mounjaro
The most frequently reported side effects of Mounjaro affect the digestive system. Very common side effects (affecting more than 1 in 10 people) include nausea, diarrhoea, vomiting, constipation, and stomach pain. These effects are often dose-related and typically improve within the first few weeks as your body adapts.
- Nausea is the most common side effect but often decreases over time
- Digestive issues like diarrhoea or constipation may occur as Mounjaro slows stomach emptying
- Stomach discomfort usually improves with smaller, more frequent meals
Common side effects (affecting up to 1 in 10 people) include indigestion, acid reflux, injection site reactions (redness or bruising), fatigue, dizziness, hair loss, and decreased appetite. Most resolve without treatment. If side effects persist beyond 4 weeks, contact your GP or the prescriber who issued your Mounjaro Kwikpen prescription.
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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 Help
Whilst rare, some side effects require immediate medical attention. Seek emergency care if you experience severe, persistent stomach pain spreading to your back (possible pancreatitis), serious allergic reactions (swelling of face, lips, tongue, or throat), changes in vision, or severe abdominal pain with vomiting that doesn't improve.
Other concerning symptoms include signs of gallbladder problems (pain in the upper stomach area, yellowing of skin or eyes) and unusual changes in heart rate or chest discomfort. Always contact your GP or NHS 111 if you're unsure whether a symptom needs medical attention. Your prescriber will assess whether continuing Mounjaro is safe for you.
Side effects can be managed through lifestyle adjustments. Eating smaller, more frequent meals, staying hydrated, and avoiding fatty or sugary foods may reduce nausea and digestive issues. Dose titration (gradually increasing your Mounjaro dose over weeks) is designed to allow your body to adjust, minimising side effects. If side effects are severe or intolerable, your prescriber can adjust your dose or discuss alternative options. Track any side effects you experience using our free progress tracker, which helps you monitor patterns and share details with your GP.
- 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.
Are Mounjaro side effects permanent?
No. Most side effects of Mounjaro are temporary and improve as your body adjusts to the medication, typically within 2–4 weeks. Digestive side effects like nausea often decrease significantly by week 4. If side effects persist, your GP can discuss dose adjustments or alternative treatments.
How does Mounjaro compare to Wegovy for side effects?
Both are GLP-1-type medications with similar common side effects (nausea, diarrhoea, constipation). Mounjaro is a dual GIP/GLP-1 agonist, whilst Wegovy is a GLP-1-only agonist. Clinical trials suggest Mounjaro may produce slightly different side effect profiles, but individual experiences vary. Your prescriber can advise which is more suitable for you.
Can I take Mounjaro if I have IBS or other digestive conditions?
If you have a pre-existing digestive condition like IBS or gastroparesis, inform your prescriber before starting Mounjaro. The medication slows stomach emptying, which may not be suitable for everyone. Your GP can assess whether Mounjaro is safe for you or recommend an alternative. Always disclose your full medical history during clinical assessment.
What should I do if I experience nausea on Mounjaro?
Start by adjusting your diet — eat smaller, more frequent meals and avoid fatty, spicy, or sugary foods. Stay hydrated and try eating plain foods like toast, rice, or soup. Nausea usually improves by week 4 as your body adjusts. If it persists or is severe, contact your prescriber; they may adjust your dose. Do not stop taking Mounjaro without medical advice.
Is it safe to use Mounjaro alongside other medications?
Mounjaro may affect how your body absorbs other oral medications, particularly contraceptive pills. Women taking oral contraceptives should add a barrier method for 4 weeks after starting Mounjaro and 4 weeks after each dose increase. Always inform your prescriber of all medications you take, including over-the-counter products and supplements.
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.