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 Cost: What You Need to Know
Mounjaro (tirzepatide) is a prescription-only weight management injection licensed in the UK. Like all medications, it can cause side effects — ranging from common digestive issues like nausea and diarrhoea to injection site reactions. This guide explains what side effects to expect, how they typically progress, and what the actual cost of Mounjaro is in the UK through both NHS specialist services and private prescription routes. Understanding both the benefits and potential side effects helps you and your GP make an informed decision about whether this medication is right for you.
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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 Cost Factors in the UK
The most common side effects of Mounjaro affect the digestive system, particularly during the dose escalation phase. Nausea is reported by more than 1 in 10 people, typically improving as your body adjusts. Diarrhoea and constipation also occur frequently — taking time to adjust diet and hydration often helps. Stomach pain, vomiting, and indigestion are less common but still possible.
Injection site reactions include redness, bruising, itching, or mild pain at the injection site — these are usually mild and temporary. Fatigue, dizziness, and hair loss have been reported. Most side effects ease after the first few weeks as your body adapts to the medication.
Cost context: Private prescriptions for Mounjaro in the UK typically range from £129–£320 per month depending on your dose (higher doses cost more). NHS access is free but availability varies significantly by region — waiting lists can be substantial in many areas, and not all regions currently fund tirzepatide through specialist weight management services.
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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: NHS vs Private Routes and What to Expect
NHS access requires referral to a tier 3 or tier 4 specialist weight management service — your GP can make this referral. NICE has approved tirzepatide for use within NHS weight management pathways, but availability depends on your local Integrated Care Board (ICB). Some regions have waiting lists exceeding 12 months.
Private access is faster: you complete an online consultation with a UK-licensed prescriber, who assesses your eligibility (BMI ≥30, or ≥27 with weight-related health conditions). If approved, your prescription is sent to a GPhC-registered pharmacy, which dispatches your KwikPen. The entire process typically takes 3–5 business days. Use our free GLP-1 weight loss calculator to estimate potential outcomes alongside your prescriber consultation. Track your 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?
Nausea, diarrhoea, constipation, and stomach pain are very common, especially during dose escalation. These typically improve within a few weeks as your body adjusts. Injection site reactions (redness, bruising) are also common but mild. More serious side effects like pancreatitis (severe stomach pain) are rare but require immediate medical attention.
Does Mounjaro cost the same on the NHS as privately?
No — NHS access is free but involves waiting lists and regional variation. Private prescriptions cost £129–£320 per month depending on your dose. Private access is faster (3–5 days) but costs more upfront. Your GP can advise on NHS options in your area.
How long does it take to get Mounjaro on the NHS?
This varies significantly by region. Some areas have waiting lists of 6–12 months or longer. Your GP can refer you to your local tier 3 weight management service, but availability depends on your Integrated Care Board (ICB). Private prescription is faster at 3–5 business days.
Can I get Mounjaro privately without seeing my GP first?
Yes — private online prescribers can assess you directly without an NHS referral. You'll complete a clinical questionnaire and health assessment, and a UK-licensed prescriber will decide if you're eligible. You do not need GP involvement, though informing your GP is advisable for continuity of care.
How does Mounjaro compare to Wegovy in terms of side effects and cost?
Both are GLP-1-type injections with similar side effect profiles (nausea, diarrhoea, constipation). Mounjaro is a dual GIP/GLP-1 agonist, Wegovy is GLP-1 only — clinical trials suggest Mounjaro may support slightly greater weight loss on average. Cost is similar: both range £129–£320 privately per month depending on dose. NHS availability varies for both medications by region.
Ready to take control of your weight loss journey?
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