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 Weight Loss Injection: Side Effects, Cost & How to Access in the UK
Mounjaro (tirzepatide) is a prescription-only weekly weight loss injection that's available in the UK through both NHS specialist services and private prescribers. This guide covers the common side effects you might experience, what it costs, and the different ways to access this dual GIP and GLP-1 receptor agonist medication. Understanding both the benefits and side effects, alongside the cost and access routes, helps you make an informed decision about whether this prescription weight loss treatment is right for you.
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Calculate Your BMI Instantly
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 to Expect
The most common side effects of Mounjaro are related to how it affects your digestive system. Nausea, diarrhoea, vomiting, and constipation are very common, affecting more than 1 in 10 people in clinical trials. Stomach pain and indigestion are also frequent, particularly when doses are being increased during the dose titration phase.
These side effects typically reduce over time as your body adjusts to the weekly weight loss injection. Most people find that gastrointestinal effects are mild to moderate and manageable with dietary adjustments. Other common side effects include fatigue, dizziness, headaches, and hair loss (alopecia). Injection site reactions like redness or bruising at the injection site are also reported but usually minor.
Serious side effects are rare but require immediate medical attention: severe, persistent stomach pain spreading to the back (possible pancreatitis), gallbladder problems, serious allergic reactions, or changes in vision. Your GP should discuss these risks before you start Mounjaro.
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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.
UK Cost and Private Access Routes
Private prescription costs for Mounjaro vary by dose and provider, typically ranging from £129 to £320 per month. Each KwikPen contains 4 weeks of doses, so costs are usually quoted as a monthly fee. Lower starting doses (2.5 mg and 5 mg) are generally less expensive than higher maintenance doses (10 mg to 15 mg), as you're getting more medication per pen.
NHS access is free through NHS specialist weight management services (tier 3/4), but availability varies significantly by region. Many areas have long waiting lists lasting months or even years. Private online prescribing offers faster access without NHS referral — you'll have a remote clinical assessment with a UK-licensed prescriber, who then issues a prescription if appropriate. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses before committing.
How to access privately: Complete an online health questionnaire with a GPhC-registered prescriber, who reviews your medical history and BMI. If approved, you'll receive a UK prescription dispensed by a regulated pharmacy. The entire process typically takes 24–48 hours. NHS route: Ask your GP for a referral to your local specialist weight management service. You'll need a BMI of 30+ or 27+ with weight-related health conditions (type 2 diabetes, high blood pressure, cardiovascular disease, sleep apnoea, or dyslipidaemia). NHS assessment is thorough and free, but waiting lists are a significant barrier in many areas. Track your weight loss progress with our free progress tracker once you start treatment.
- 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 are the most common side effects of Mounjaro when starting?
Nausea, diarrhoea, vomiting, and constipation are very common, affecting more than 1 in 10 people. These typically ease over time as your body adjusts during the dose titration process. Most people find side effects manageable with dietary changes and staying hydrated.
How much does Mounjaro cost on the NHS in the UK?
Mounjaro is free on the NHS when prescribed through specialist tier 3/4 weight management services. However, NHS access is limited and varies by region — many areas have waiting lists of several months. Availability depends on your local Integrated Care Board.
What's the difference between NHS and private Mounjaro access?
NHS access is free but involves referral, assessment, and potentially long waiting lists. Private access is faster (24–48 hours) and doesn't require NHS referral, but costs £129–£320 per month depending on dose. Both routes require a clinical assessment and prescription.
Can I switch between doses if side effects are too severe?
Yes. If side effects are problematic, your prescriber can reduce your dose or slow the titration schedule. Dose titration typically increases every 4 weeks, so discussing concerns early allows your prescriber to adjust. Never stop or change doses without guidance from your healthcare provider.
What should I do if I experience severe side effects like severe stomach pain?
Seek immediate medical attention. Severe stomach pain spreading to the back could indicate pancreatitis or gallbladder problems. Contact your GP, call 111, or attend A&E. Always inform emergency staff that you're taking Mounjaro. Report any serious side effects to your prescriber immediately — they may recommend stopping 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.