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 You Need to Know in the UK
Mounjaro (tirzepatide) is a prescription-only weight loss injection licensed in the UK that works by mimicking two natural hormones to help regulate appetite and blood sugar. Like all medicines, Mounjaro can cause side effects — most commonly nausea, diarrhoea, and stomach discomfort, especially when first starting treatment or increasing doses. Understanding what side effects are possible and how the NHS prescribes this medication can help you make informed decisions about your weight management options.
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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 the NHS Expects
Mounjaro's side effects are generally dose-related and often improve over time as your body adjusts. Very common side effects (affecting more than 1 in 10 people) include nausea, diarrhoea, vomiting, constipation, and abdominal pain — most often reported in the first few weeks of treatment or during dose titration.
Common side effects include indigestion, acid reflux (GORD), injection site reactions, fatigue, dizziness, and hair loss. These typically occur in fewer than 1 in 10 users.
The NHS notes that most people experience mild to moderate gastrointestinal symptoms that settle within days to weeks. Staying hydrated, eating smaller meals, and avoiding fatty foods can help manage these effects. Your GP or tier 3 weight management service will monitor you throughout treatment and can adjust your dose if side effects are troublesome. Serious side effects requiring immediate medical attention include severe stomach pain (possible pancreatitis), gallbladder problems, or allergic reactions.
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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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NHS Prescribing and Eligibility for Mounjaro in the UK
NICE has approved tirzepatide (Mounjaro) as an option for weight management in the UK, available through NHS specialist tier 3 weight management services. Eligibility typically includes adults with a BMI of 30 kg/m² or above, or 25–29.9 kg/m² with weight-related conditions like type 2 diabetes, high blood pressure, or sleep apnoea.
NHS prescribing varies significantly by region — some integrated care boards have lengthy waiting lists while others may have limited availability. If you're interested in Mounjaro, your GP can refer you to your local tier 3 service for assessment. Check your BMI using our free BMI calculator to see if you fall within typical eligibility ranges. Private prescribing routes are also available without NHS referral, often with faster access but at personal cost.
Mounjaro vs Wegovy comparison: Both are once-weekly weight loss injections licensed in the UK. Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) works via GLP-1 alone. Clinical trial data shows Mounjaro may produce slightly greater average weight loss, but individual results vary significantly. Side effect profiles are similar, though Mounjaro and Wegovy use different starting doses. Your GP or prescriber will assess which medication is most suitable based on your health history and preferences.
- 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 available on the NHS in the UK?
Yes, NICE has approved tirzepatide (Mounjaro) for NHS use. However, it's typically prescribed through specialist tier 3 weight management services, not standard GP prescriptions. Availability varies by region, and waiting lists can be lengthy. Your GP can refer you for assessment if you meet eligibility criteria.
What are the most common side effects of Mounjaro?
The most common side effects are nausea, diarrhoea, vomiting, and stomach discomfort — affecting more than 1 in 10 people. These usually occur when starting treatment or during dose increases and often settle within days to weeks. Staying hydrated and eating smaller meals can help manage these symptoms.
When should I seek medical help for Mounjaro side effects?
Seek immediate medical attention if you experience severe, persistent stomach pain (possible pancreatitis), gallbladder pain, serious allergic reactions with swelling of the face or throat, or unexplained changes in vision. Contact your GP for any side effects that are severe, worsening, or persist beyond a few weeks.
How is Mounjaro prescribing different between NHS and private routes in the UK?
NHS Mounjaro comes through tier 3 specialist services and may involve waiting lists. Private prescribing is available via UK-licensed online prescribers and GPhC-registered pharmacies, often with faster access but at personal expense. Both routes require a clinical assessment before prescription.
How does Mounjaro's dose titration work and does this affect side effects?
Mounjaro starts at 2.5 mg weekly and increases gradually every 4 weeks (5 mg, 7.5 mg, 10 mg, 12.5 mg, 15 mg maximum). Side effects are often worse during dose increases, then settle as your body adjusts. Your prescriber will slow or pause dose increases if side effects are too troublesome.
Ready to take control of your weight loss journey?
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