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 Can Mounjaro Cause — Side Effects Guide
Mounjaro (tirzepatide) is a prescription-only weight management injection that may cause side effects in some people. The most common effects include nausea, diarrhoea, and stomach pain, particularly when starting or increasing doses. Most side effects are temporary and improve over time as your body adjusts to the medication. Understanding what Mounjaro can cause helps you decide if it's right for you and know when to seek medical advice. Always discuss potential side effects with your prescriber before starting treatment.
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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 in the UK
According to the Summary of Product Characteristics and clinical trial data, very common side effects (affecting more than 1 in 10 people) include nausea, diarrhoea, vomiting, constipation, and abdominal pain. Common side effects (affecting up to 1 in 10 people) include indigestion, acid reflux, injection site reactions, fatigue, dizziness, and hair loss. Most gastrointestinal side effects occur early in treatment or when increasing your dose and often settle within weeks as your body adapts.
- Nausea typically peaks in the first 2–4 weeks and may improve as you continue treatment
- Injection site reactions are usually mild redness, bruising, or discomfort that resolve quickly
- Hair loss is temporary and typically reversible after stopping the medication
- Dizziness may occur if you're not eating enough during treatment
If side effects are severe or persistent, your prescriber can adjust your dose or provide symptom management strategies. Our GLP-1 weight loss calculator can help you understand potential outcomes and discuss realistic expectations with your healthcare team.
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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 Help
Serious side effects are rare but require immediate medical attention. Seek urgent care if you experience severe, persistent stomach pain radiating to the back (possible pancreatitis), serious allergic reactions (swelling of face, lips, or throat), changes in vision, rapid or concerning changes in heart rate, or signs of gallbladder problems (severe upper stomach pain). Although these are uncommon, they're important to recognise. If you experience any symptoms that worry you, contact your GP, NHS 111, or attend A&E immediately.
Your prescriber will assess your personal risk factors during your initial clinical assessment. Use our free BMI calculator and weight converter to discuss your current health context with your GP or weight management specialist before starting Mounjaro.
- 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.
What is the most common side effect of Mounjaro?
Nausea is the most frequently reported side effect, affecting more than 1 in 10 people. It's usually worst in the first few weeks and often improves as your body adjusts. Eating smaller meals, staying hydrated, and avoiding fatty foods may help. Always speak to your prescriber if nausea is severe or interfering with your daily life.
How long do Mounjaro side effects last?
Most gastrointestinal side effects settle within 2–4 weeks as your body adapts to the medication. Injection site reactions typically resolve within hours to days. Some effects like hair loss may take longer to reverse after stopping treatment. Individual timelines vary — your prescriber can advise based on your experience.
Can I take Mounjaro if I have a sensitive stomach?
If you have a history of stomach problems, gastroparesis, or severe digestive issues, discuss this with your prescriber before starting. Mounjaro may not be suitable for you, or your prescriber may recommend additional monitoring. Always mention your full medical history during your clinical assessment.
What's the difference between Mounjaro and Wegovy side effects?
Both are GLP-1-type medications with similar common side effects (nausea, diarrhoea, stomach pain). Mounjaro is a dual GIP/GLP-1 agonist and may work differently in your body. Individual responses vary significantly. Your prescriber can compare options based on your medical history and preferences.
Should I stop Mounjaro if I experience side effects?
Don't stop suddenly without speaking to your prescriber — they may adjust your dose, slow your titration, or offer strategies to manage symptoms. Stopping abruptly can cause weight regain. Your prescriber will help you decide whether to continue, adjust, or try a different approach based on your individual situation.
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.