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
Mounjaro Side Effects — UK Health Guide
Mounjaro (tirzepatide) is a once-weekly weight loss injection licensed in the UK for adults with obesity or overweight with weight-related health conditions. Like all prescription medications, it can cause side effects. Most are mild and improve over time, but it's important to know what to expect. This guide covers the most common side effects, when they typically occur, and what NHS guidance says about managing them safely.
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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 What NHS Data Shows
Clinical trials and UK prescribing data show that nausea, diarrhoea, vomiting, and constipation are the most frequently reported side effects during the early weeks of treatment. These tend to be gastrointestinal (stomach-related) because tirzepatide slows how quickly food leaves your stomach. Most people find these effects improve or resolve within 4–8 weeks as the body adjusts.
- Very common (more than 1 in 10 users): Nausea, diarrhoea, vomiting, constipation, abdominal pain
- Common (up to 1 in 10 users): Indigestion, acid reflux, headache, fatigue, dizziness, injection site reactions, hair loss
- Important: Side effects are usually mild to moderate and typically improve as the body adjusts to the medication
If you experience side effects, speak to your GP or prescriber before stopping the medication. Your dose may be adjusted, or you may benefit from temporary dietary changes to manage symptoms.
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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.
NHS Prescribing and When to Seek Urgent Help
The NHS tier 3 specialist weight management services monitor patients carefully during treatment. Seek immediate medical attention if you experience: severe, persistent stomach pain spreading to the back (possible pancreatitis), serious allergic reactions (swelling of face, lips, throat), sudden changes in vision, or gallbladder problems (pain in upper stomach area). These are rare but require urgent care.
Most side effects are manageable and do not require stopping treatment. Eating smaller, lighter meals; staying hydrated; and taking the medication consistently can help. Use our GLP-1 weight loss calculator to understand expected outcomes, and track your progress with our free progress tracker to monitor how you feel week to week.
- 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 are temporary and improve within 4–8 weeks as your body adjusts to the medication. Nausea, diarrhoea, and constipation typically reduce significantly by weeks 8–12. If side effects persist, your GP or prescriber can adjust your dose or discuss alternative options.
How does Mounjaro compare to Wegovy for side effects?
Both medications cause similar gastrointestinal side effects because they work on appetite regulation. Mounjaro uses a dual GIP and GLP-1 mechanism, while Wegovy is GLP-1 only. Individual tolerability varies, and some people find one easier to tolerate than the other. Your prescriber can help decide which is most suitable for you.
Can I take anti-nausea medication with Mounjaro?
Always discuss this with your GP or prescriber before taking any additional medication. Some anti-nausea treatments may interact with Mounjaro or affect how well it works. Your healthcare professional can recommend safe options if needed.
What should I eat if Mounjaro causes diarrhoea or constipation?
Adjust your diet based on which side effect you experience. For diarrhoea: eat bland, easily digestible foods, stay hydrated, and avoid high-fat meals. For constipation: increase soluble fibre gradually (oats, fruit), drink plenty of water, and consider gentle exercise. Smaller, more frequent meals often help too.
Will Mounjaro side effects stop if I stop taking it?
Most side effects resolve within days to weeks of stopping the medication. However, stopping without medical guidance may affect your weight management plan. Always consult your GP or prescriber before discontinuing, as they can discuss alternatives or strategies to manage side effects while continuing treatment.
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.