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, Cost & How to Access in the UK
Mounjaro (tirzepatide) is a prescription-only weight management injection that's become increasingly popular as a "fat jab" in the UK. Before starting, it's important to understand both the potential side effects you might experience and the different ways to access this medication — whether through the NHS or private prescription. This guide covers what you need to know about how Mounjaro works, common side effects, eligibility criteria, and the costs involved depending on your access route.
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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
Most people taking Mounjaro experience some gastrointestinal side effects, particularly when starting or increasing doses. Very common side effects include nausea, diarrhoea, vomiting, constipation, and stomach pain — these typically improve over time as your body adjusts. Common side effects include indigestion, acid reflux, injection site reactions (redness or bruising), fatigue, dizziness, and hair loss. Most side effects are mild to moderate and temporary.
Serious side effects are rare but require immediate medical attention: severe stomach pain spreading to the back (possible pancreatitis), gallbladder problems, serious allergic reactions, or changes in vision. If you take oral contraceptives, Mounjaro may reduce their effectiveness for 4 weeks after starting or each dose increase — discuss additional contraception with your prescriber. Always inform your GP or prescriber of all medications and health conditions before starting.
Side effect severity often depends on your starting dose and how quickly it increases. Use our free GLP-1 weight loss calculator to explore how different doses might work for you.
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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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How to Access Mounjaro in the UK — NHS vs Private
NHS access: NICE has approved tirzepatide for NHS use, but availability is limited to specialist tier 3 weight management services. You'll need a GP referral, and access varies significantly by region — some areas have lengthy waiting lists or restricted eligibility criteria. NHS prescription is free once you're in the system.
Private access: You can obtain Mounjaro privately through a UK-licensed prescriber via online consultation, typically costing £129–£320 per month depending on dose. A GPhC-registered pharmacy dispenses your prescription. The online consultation process involves completing a health questionnaire, and a prescriber assesses whether Mounjaro is suitable for you based on your BMI, health history, and any weight-related conditions. Private access is faster but requires self-funding. Use our BMI calculator above to check your eligibility status.
What the online consultation includes: A detailed health questionnaire covering your weight history, current medications, allergies, blood pressure, and any existing health conditions like diabetes or heart disease. The prescriber reviews this information and may request additional details. If approved, your prescription is sent to a pharmacy, which prepares and ships your Mounjaro pens. Your first order typically arrives within 2–5 working days. You'll receive guidance on injection technique, storage (refrigerate unopened pens at 2–8°C), and how to escalate your dose every 4 weeks. Many private providers offer ongoing support via your patient account.
- 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's the difference between Mounjaro and Wegovy?
Both are prescription-only weight loss injections. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1 agonist only. Clinical trials suggest Mounjaro may produce slightly greater average weight loss (~21% vs ~15% body weight), but individual results vary significantly. Both require prescription and clinical assessment.
Can I get Mounjaro on the NHS?
Yes — NICE has approved tirzepatide for NHS weight management services. However, access is through specialist tier 3 weight management services, not routine GP prescription. You'll need a referral and meet specific eligibility criteria (BMI ≥30 or BMI 27–29.9 with weight-related health conditions). Availability and waiting times vary greatly by region and local ICB funding.
How much does Mounjaro cost privately in the UK?
Private Mounjaro typically costs £129–£320 per month depending on your dose. Starting doses (2.5 mg) are cheaper; maximum doses (15 mg) are more expensive. One KwikPen contains 4 weekly doses. Prices vary by provider. Some private clinics offer subscription discounts or bundle pricing for multiple months.
What happens during an online Mounjaro consultation?
You complete a detailed health questionnaire covering weight, medical history, current medications, allergies, and comorbidities. A UK-licensed prescriber reviews this and may contact you for clarification. If approved, they issue a prescription to a GPhC-registered pharmacy. Your pens are prepared and shipped to you, usually within 2–5 working days. Some providers include follow-up support.
Will Mounjaro side effects stop me from using it?
Most side effects are mild, temporary, and improve as your body adjusts — especially within the first 4 weeks. Starting at the lowest dose (2.5 mg) and escalating gradually minimises side effects. Common nausea or stomach discomfort can be managed with dietary changes (smaller meals, avoiding fatty foods). Serious side effects are rare. Discuss any concerns with your prescriber before starting.
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.