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 in the UK: Cost and Access Guide
Mounjaro (tirzepatide) is a prescription-only weight loss injection licensed in the UK, but understanding both its side effects and how to access it—along with the costs involved—is essential before considering treatment. Whether you're exploring NHS or private routes, it's important to know what side effects to expect and what the financial commitment looks like. This guide covers everything you need to know about Mounjaro availability and affordability in the UK.
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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 Side Effects and What They Mean for Access
Mounjaro's most common side effects include nausea, diarrhoea, vomiting, and constipation—particularly during the dose escalation phase. These typically improve as your body adapts, usually within the first 2-4 weeks at each dose level. Other frequent effects include indigestion, fatigue, dizziness, injection site reactions (redness or bruising), and temporary hair loss. The KwikPen injection itself is straightforward to use once weekly.
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. Before accessing Mounjaro—whether through the NHS or privately—a clinical assessment will identify whether you have risk factors that contraindicate treatment.
- Gastrointestinal side effects often settle by weeks 3-4 at each dose
- Drinking water, eating smaller meals, and eating slowly can help manage nausea
- If side effects persist or worsen, your prescriber can slow your dose escalation
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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 vs Private: Access Routes and Cost Context
NHS access: Mounjaro is available through NHS specialist weight management services (tier 3), but access is highly regional and waiting lists can be several months or longer depending on your area. You'll need a GP referral to a local specialist service, and eligibility assessment happens at the service. NHS prescription costs follow standard NHS charges (currently £10.65 per item in England, or free if exempt).
Private access: You can obtain a prescription privately through a UK-licensed online prescriber following a confidential consultation. Private costs typically range from £129 to £320+ per month depending on your dose level and provider. After initial assessment, ongoing prescriptions and KwikPen dispensing happen through GPhC-registered pharmacies. This route bypasses NHS waiting lists but requires private payment. Use our free GLP-1 weight loss calculator to estimate potential outcomes at different doses.
- 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.
How much does Mounjaro cost privately in the UK?
Private Mounjaro costs range from approximately £129–£320+ per month depending on your dose. Starting doses (2.5–5mg) are typically £129–£160, while higher maintenance doses (10–15mg) cost £220–£320+. Prices vary between providers. Most private providers offer monthly subscription models with prescriptions renewed after online check-ins.
Can I get Mounjaro free on the NHS?
Yes, Mounjaro is available on the NHS through specialist weight management services, where you'd pay only the standard prescription charge (£10.65 per item in England, or free if exempt). However, availability varies significantly by region, and NHS waiting lists can be several months. Ask your GP about referral to your local tier 3 weight management service.
What side effects might prevent me accessing Mounjaro?
A clinical assessment will identify contraindications. You cannot take Mounjaro if pregnant, breastfeeding, allergic to tirzepatide, under 18, or have severe gastroparesis. A history of pancreatitis, gallbladder disease, or diabetic retinopathy requires careful medical consideration. Your prescriber will review your full health history before approving treatment.
How long does the NHS assessment process take?
NHS referral to initial assessment typically takes 2–6 months depending on your regional specialist service's capacity. Once assessed and approved, you may wait further for medication to start. Private online assessment usually happens within days, with medication arriving within 1–2 weeks. Check your local ICB waiting times through your GP.
Can I switch from private Mounjaro to NHS?
Potentially, yes. If you've been on private Mounjaro and wish to switch to NHS, discuss this with your GP and ask for referral to your local specialist weight management service. They'll assess whether to continue the same dose or manage transition. Some services may accept continuity of care requests, but policies vary regionally.
Ready to take control of your weight loss journey?
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