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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Mounjaro Side Effects: What to Expect in the UK
Mounjaro (tirzepatide), often called a "weight loss injections" in the UK, is a prescription-only weight management injection licensed by the MHRA. Like all medications, it can cause side effects. Most are mild and improve over time, but it's important to understand what to expect before starting treatment. This guide covers Mounjaro side effects in an NHS and UK context, helping you make an informed decision alongside your GP or prescriber.
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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 Mounjaro Side Effects and What They Mean
Very common side effects (affecting more than 1 in 10 people) include nausea, diarrhoea, vomiting, constipation, and stomach pain. Nausea is typically worst in the first 2–4 weeks and often improves as your body adjusts. Diarrhoea and constipation can alternate as your digestive system responds to the medication.
Common side effects include indigestion, acid reflux, injection site reactions (redness or bruising), fatigue, dizziness, and hair loss. Hair loss is usually temporary and linked to gradual weight management rather than the medication itself.
- Nausea: eat small, frequent meals; avoid fatty or spicy foods
- Constipation: increase water intake and fibre gradually
- Injection site reactions: rotate injection sites weekly
- Fatigue: ensure adequate sleep and nutrition during weight loss
Most side effects settle within 4–6 weeks. Your Mounjaro KwikPen delivers a precise dose once weekly, allowing your body time to adjust between injections. If side effects persist or worsen, contact your prescriber — dose adjustment or timing changes may help.
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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.
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Serious Side Effects and When to Seek Medical Help
Serious side effects requiring urgent medical attention are rare but important to recognise. Seek immediate help if you experience severe, persistent stomach pain spreading to your back (possible pancreatitis), serious allergic reactions (swelling of face, lips, tongue, or throat), gallbladder problems (upper stomach pain), or unexplained changes in vision.
These complications are uncommon, but because Mounjaro is subject to MHRA additional monitoring (Black Triangle ▼ status), report any new or unusual symptoms to your GP. You can also report side effects to the Yellow Card scheme — the UK's official drug safety reporting system. Never ignore warning signs, especially severe abdominal pain, as pancreatitis requires emergency treatment. Your safety is the priority.
---COL_J_TOOL_REF--- Calculate your personal BMI using our free BMI calculator above to understand your weight context before discussing Mounjaro eligibility 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.
Is Mounjaro approved by the NHS in the UK?
Yes, Mounjaro (tirzepatide) is MHRA-approved and NICE has recommended it for use within NHS specialist weight management services (tier 3). However, NHS availability varies significantly by region and waiting lists can be lengthy. You can also access Mounjaro privately through a UK-licensed prescriber following an online clinical assessment.
What's the difference between Mounjaro and Wegovy?
Both are once-weekly injections for weight management in the UK. Mounjaro is a dual GIP and GLP-1 agonist with a maximum dose of 15mg; Wegovy is a GLP-1 agonist only with a maximum dose of 2.4mg. Mounjaro started at a lower dose (2.5mg) than Wegovy (0.25mg). Clinical trials showed slightly higher average weight loss with Mounjaro, but individual results vary significantly.
How long do Mounjaro side effects last?
Most side effects are mild and temporary, improving within 4–6 weeks as your body adjusts. Nausea is usually worst in weeks 1–2 and settles by week 4. Digestive symptoms often improve over time. If side effects persist beyond 6 weeks, contact your prescriber — you may need a slower dose escalation or review of your nutrition and hydration.
Can I report Mounjaro side effects to the NHS?
Yes. If you experience side effects, tell your GP or prescriber immediately. They can document it and, if necessary, report it to the MHRA's Yellow Card scheme — the UK's official medicine safety reporting system. You can also report directly at www.yellowcard.mhra.gov.uk. Reporting helps monitor drug safety across the UK population.
What should I do if I develop severe stomach pain while taking Mounjaro?
Severe stomach pain spreading to your back is a medical emergency and may indicate pancreatitis. Call 999 or go to A&E immediately. Do not wait to contact your GP. Pancreatitis requires urgent hospital assessment. Always inform medical staff that you're taking Mounjaro so they can assess you fully and safely.
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