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 and UK Access Guide
Mounjaro (tirzepatide) is a weekly weight loss injection that may help you lose weight when combined with diet and exercise changes. Before starting this prescription-only medicine, it's important to understand the potential side effects and how to access it in the UK. This guide covers what side effects to expect, how much Mounjaro costs privately, and the differences between NHS and private prescription routes. Whether you're considering this weekly weight loss injection through your GP or exploring private options, knowing the full picture helps you make an informed decision about your weight management.
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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.
Mounjaro Side Effects: What to Expect
Common side effects of Mounjaro typically occur during the dose titration phase as your body adjusts to the medication. The most frequently reported side effects include nausea, diarrhoea, vomiting, and constipation, particularly in the first few weeks of treatment or when increasing your dose.
- Very common (affecting more than 1 in 10 people): nausea, diarrhoea, vomiting, constipation, and stomach pain
- Common (affecting up to 1 in 10 people): indigestion, acid reflux, injection site reactions, tiredness, dizziness, and hair loss
- Less common but serious: severe stomach pain spreading to the back (possible pancreatitis), gallbladder problems, or serious allergic reactions
Most gastrointestinal side effects improve over time as your body adapts. Staying hydrated, eating smaller meals, and avoiding fatty foods can help manage nausea. If side effects are severe or persist beyond a few weeks, contact your prescriber — they may adjust your dose or recommend additional support.
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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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Accessing Mounjaro in the UK: NHS vs Private Cost
NHS access: Mounjaro is available through NHS specialist weight management services (tier 3/4), but availability varies significantly by region and waiting lists can be lengthy. You'll need a referral from your GP, and access depends on meeting specific eligibility criteria and your local Integrated Care Board's commissioning decisions.
Private prescription cost: Privately, Mounjaro typically costs between £130–£300 per month depending on your dose (the KwikPen containing 4 weekly injections). Private prescriptions are issued following an online clinical assessment with a UK-licensed prescriber through registered pharmacies. This route offers faster access without NHS waiting lists, though it is not funded by the NHS. Calculate your potential costs using our free GLP-1 weight loss calculator to compare doses and estimate your investment in treatment.
Online prescription process: If pursuing private treatment, you'll complete a brief health questionnaire, have a consultation (often via video call) with a licensed prescriber, and receive your prescription if deemed clinically appropriate. The KwikPen is then dispatched by a GPhC-registered pharmacy. Important: prescription-only medicines require a proper clinical assessment — be cautious of services that prescribe without thorough screening. Track your progress once you start using our free progress tracker, logging your weight and how you're tolerating the medication.
- 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 Mounjaro side effects?
The most common side effects are nausea, diarrhoea, vomiting, and constipation, especially during dose increases. These typically improve over time as your body adjusts. Staying hydrated and eating smaller meals can help. If side effects are severe, contact your prescriber.
How much does Mounjaro cost privately in the UK?
Private Mounjaro typically costs £130–£300 per month depending on your dose. Each KwikPen contains 4 weekly injections. Prices vary by pharmacy and dose strength. Always check with your chosen provider for current pricing before starting treatment.
Can I get Mounjaro on the NHS in the UK?
Mounjaro is available through NHS specialist weight management services, but access varies by region and requires a GP referral. NICE has approved it for NHS use, but availability depends on your local Integrated Care Board's commissioning and current waiting lists can be very long.
What's the difference between getting Mounjaro through NHS vs private prescription?
NHS access is free but involves GP referral, specialist assessment, and potential waiting lists. Private prescriptions offer faster access following an online clinical assessment with a licensed prescriber, but you pay the full cost. Both routes require a proper prescription and clinical evaluation.
Is Mounjaro the same as Wegovy?
No. Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, while Wegovy (semaglutide) is a GLP-1 agonist only. Both are weekly injections licensed for weight management in the UK, but they work slightly differently. Clinical trials show both may help with weight loss — individual results vary. Your prescriber can advise which is more suitable for you.
Ready to take control of your weight loss journey?
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